Method and apparatus for curtain coating

ABSTRACT

A curtain coating method including forming a curtain including a coating material. A coated web is formed by coating a moving primary web with the coating material. The curtain and the primary web meet at a contact line. A primary pattern is projected on a material layer. The material layer moves with respect to the contact line. The primary pattern includes a first pointer feature. A first image is captured. The first image includes a first sub-image of the first pointer feature. The position of the first sub-image is in the first image is detected. Distance information is provided by comparing the detected position of the first sub-image with a reference position.

FIELD

The present invention relates to curtain coating.

BACKGROUND

A coated web may be produced by a curtain coating method. The curtaincoating method may comprise forming a curtain of a fluid, which fallsonto a primary web in order to produce a coated web. The curtain maysometimes have a void portion, and the produced web produced by thecurtain coating method may need to be rejected due to inferior quality.

SUMMARY

Some variations may relate to a method for curtain coating. Somevariations may relate to an apparatus for curtain coating. Somevariations may relate to a computer program for monitoring curtaincoating. Some variations may relate to a computer program product formonitoring curtain coating. Some variations may relate to a combinationof a coated web and auxiliary data.

According to a first aspect, there is provided a method, comprising:

-   -   forming a curtain (CUR1), which comprises a coating material        (ADH0),    -   forming a coated web (WEB1) by coating a moving primary web        (WEB0) with the coating material (ADH0), wherein the curtain        (CUR1) and the primary web (WEB1) meet at a contact line (CL1),    -   projecting a primary pattern (PTRN1) on a material layer (CUR1,        WEB0, WEB1), whose material moves with respect to the contact        line (CL1), wherein the primary pattern (PTRN1) comprises a        first pointer feature (D_(A,k)),    -   capturing a first image (FRAME1), which comprises a first        sub-image (SD_(A,k)) of the first pointer feature (D_(A,k)),    -   detecting the position (u_(k)(t₁)) of the first sub-image        (SD_(A,k)) in the first image (FRAME1), and    -   providing distance information (z(t),L_(k)(t)) by comparing the        detected position (u_(k)(t₁)) of the first sub-image (SD_(A,k))        with a reference position (REF1_(k)).

According to a second aspect, there is provided an apparatus (900),comprising:

-   -   one or more rolls (550) arranged to move a primary web (WEB0),    -   a distributor unit (510) arranged to form a curtain (CUR1),        which comprises a coating material (ADH0), and to form a coated        web (WEB1) by coating the primary web (WEB0) with the coating        material (ADH0) such that the curtain (CUR1) meets the primary        web (WEB0) at a contact line (CL1),    -   a projection unit (200) arranged to project a primary pattern        (PTRN1) on a material layer (ADH0, WEB0, WEB1), whose material        is arranged to move with respect to the contact line (CL1),        wherein the primary pattern (PTRN1) comprises a first pointer        feature (D_(A,k)),    -   an imaging unit (100) arranged to capture a first image        (FRAME1), which comprises a first sub-image (SD_(A,k)) of the        first pointer feature (D_(A,k)), and    -   one or more data processors (CNT1) configured to provide        distance information (z(t),L_(k)(t)) by comparing the position        (u_(k)(t₁)) of the first sub-image (SD_(A,k)) with a reference        position (REF1_(k)).

According to a further aspect, there is provided a method, comprising:

-   -   forming a curtain (CUR1), which comprises a coating material        (ADH0),    -   forming a coated web (WEB1) by coating a moving primary web        (WEB0) with the coating material (ADH0),    -   projecting a primary pattern (PTRN1) on the curtain (CUR1),        wherein the primary pattern (PTRN1) comprises a first pointer        feature (D_(A,k)),    -   capturing a first image (FRAME1), which comprises a first        sub-image (SD_(A,k)) of the first pointer feature (D_(A,k)),    -   detecting the position (u_(k)(t₁)) of the first sub-image        (SD_(A,k)) in the first image (FRAME1), and    -   providing distance information (z(t),L_(k)(t)) by comparing the        detected position (u_(k)(t₁)) of the first sub-image (SD_(A,k))        with a reference position (REF1_(k)).

According to a further aspect, there is provided an apparatus (900),comprising:

-   -   one or more rolls (550) arranged to move a primary web (WEB0),    -   a distributor unit (510) arranged to form a curtain (CUR1),        which comprises a coating material (ADH0), and to form a coated        web (WEB1) by coating the moving primary web (WEB0) with the        coating material (ADH0),    -   a projection unit (200) arranged to project a primary pattern        (PTRN1) on the curtain (CUR1) such that the primary pattern        (PTRN1) comprises a first pointer feature (D_(A,k)),    -   an imaging unit (100) arranged to capture a first image        (FRAME1), which comprises a first sub-image (SD_(A,k)) of the        first pointer feature (D_(A,k)), and    -   one or more data processors (CNT1) configured to provide        distance information (z(t),L_(k)(t)) by comparing the position        (u_(k)(t₁)) of the first sub-image (SD_(A,k)) with a reference        position (REF1_(k)).

According to a further aspect, there is provided a combination of a web(WEB1, WEB3) and data (DATA1), wherein the web (WEB1, WEB3) has beenproduced by curtain coating, and the data (DATA1) comprises defect data,which indicates the positions (x′_(j), y′_(j)) of one or more defectportions (F_(j), F_(j+1)) of the web (WEB1,WEB3).

A method for curtain coating may comprise forming a curtain, whichcomprises a coating material. The coating material of the curtain may besubstantially in a liquid state. A coated web may be formed by coating amoving primary web with the coating material of the curtain. The coatingmaterial of the curtain may fall downwards until it touches the movingprimary web. In particular, the coating material may be brought intocontact with the primary web at a contact line.

The method may include providing several material layers, which aremoving with respect to contact line during the curtain coating. Inparticular, the primary web, the coated web, and the material of thecurtain may move with respect to contact line. The material layers mayhave deflected portions, deformed portions, and/or void portions, whichmay have an adverse effect on the quality of the coated web. Thematerial layers may be monitored by determining distance information.The method may comprise projecting one or more pointer features on amaterial layer, capturing an image, which comprises sub-images of thepointer features, and determining distance information from thepositions of the sub-images of the pointer features.

Using the distance information may allow optimization of operatingparameters of the coating process, which in turn may allow e.g.increasing the velocity of the primary web and/or may allow reducing theprobability of producing defective coated web. For example, thethree-dimensional shape of the curtain may be stabilized by using one ormore precisely controlled gas flows, wherein the flow rates of the gasflows may be adjusted based on the distance information.

In an embodiment, using the distance information may allow smartdie-cutting, where a coated web may be effectively utilized also whenthe coated web comprises defective portions. This may minimize theamount of rejected material.

Using the method may allow producing a coated web, where the coatinglayer has a substantially constant thickness, i.e. the spatialvariations may be very small. This may allow minimizing the amount ofthe coating material used for producing the coated web.

In an embodiment, the method may comprise forming a label web or a labellaminate web in order to produce substantially transparent ortranslucent labels. Thanks to the method, the produced web may havespatially uniform visual appearance. In an embodiment, the thickness ofthe coating layer may be minimized, which in turn may improve thelight-transmitting properties of the web.

In an embodiment, the instantaneous shape of the curtain may bemonitored by simultaneously projecting a plurality of pointer featureson the curtain, and by analyzing the images of the pointer features. Inan embodiment, the three-dimensional instantaneous shape of the curtainmay be monitored by simultaneously projecting a plurality of pointerfeatures on the curtain, and by analyzing the images of the pointerfeatures.

In an embodiment, an image of the curtain may be captured and analyzedin order to determine whether the curtain has a fully continuous surfaceor whether the curtain has one or more void portions. A void may bedetected e.g. by using pixel value thresholding. The image of thecurtain may be a greyscale image or a color image. The greyscale imageor the color image may be analyzed e.g. by pixel value thresholding inorder to detect a void. The reliability of detecting a void may beimproved when the information obtained by the thresholding issupplemented with distance information. If the presence of the voidscannot be detected reliably, this may cause wasteful rejection of thecoated web and/or this may cause quality problems.

A coated web produced by a curtain coating process may be associatedwith defect data obtained by monitoring the curtain during the curtaincoating process. The defective portions of the coated web may besubsequently identified by using the defect data. The defective portionsmay be identified and cut away from the web. The defective portions maybe rejected or used for less demanding applications. The remainingintact portions may be used normally e.g. for producing adhesive labels.

In an embodiment, monitoring the curtain may also be disturbed by aforeign object, which temporarily blocks the field of view of amonitoring unit. The foreign object may be e.g. a person who isperforming a maintenance operation, or a tool held by the person. In anembodiment, the presence of the foreign object may be detected by usingdistance information. A coated web produced by a curtain coating processmay be associated with surveillance data obtained by monitoring thecurtain during the curtain coating process. The surveillance data maye.g. indicate when the presence of a foreign object was detected. Thesurveillance data may be provided e.g. based on distance information.The uninspected portions of the coated web may be subsequentlyidentified by using the defect data. For example, the uninspectedportions may be identified and treated separately. For example, theuninspected portions may be rejected, or used for a less demandingapplication.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following examples, several variations will be described in moredetail with reference to the appended drawings, in which:

FIG. 1 shows, by way of example, in a side view, an apparatus forproducing a coated web by curtain coating,

FIG. 2 a shows, by way of example, in a three dimensional view, a labellaminate web,

FIG. 2 b shows, by way of example, in a three dimensional view, a labelattached to an item,

FIG. 2 c shows, by way of example, in a three dimensional view, a labellaminate web,

FIG. 2 d shows, by way of example, in a three dimensional view, a labelattached to an item,

FIG. 3 a shows, by way of example, in a side view, producing a coatedweb by curtain coating,

FIG. 3 b shows, by way of example, in a side view, the contact linewhere the curtain meets the primary web,

FIG. 4 shows, by way of example, in a three dimensional view, a curtaincoating apparatus,

FIG. 5 a shows, by way of example, in a three dimensional view,monitoring a curtain by an imaging unit,

FIG. 5 b shows, by way of example, in a top view, monitoring the curtainby an imaging unit,

FIG. 5 c shows, by way of example, a digital image of the curtain,

FIG. 6 shows, by way of example, in a three dimensional view, monitoringthe curtain by a monitoring unit,

FIG. 7 a shows, by way of example, in a top view, monitoring the curtainby a monitoring unit,

FIG. 7 b shows, by way of example, in a top view, capturing an image ofa projected feature in a reference state,

FIG. 7 c shows, by way of example, in a top view, capturing an image ofa projected feature in case of a deformed curtain,

FIG. 7 d shows, by way of example, in a three dimensional view, adeformed curtain,

FIG. 8 a shows, by way of example, in a side view, a plurality offeatures projected on the curtain,

FIG. 8 b shows, by way of example, an image of the features projected onthe curtain in the reference state,

FIG. 8 c shows, by way of example, an image of the features projected onthe curtain in case of a deformed curtain,

FIG. 8 d shows, by way of example, in a three dimensional view, aforeign object positioned between the monitoring unit and the curtain,

FIG. 9 a shows, by way of example, in a side view, several sub-patternsprojected on the curtain,

FIG. 9 b shows, by way of example, an image of the sub-patternsprojected on the curtain in the reference situation,

FIG. 9 c shows, by way of example, an image of the sub-patternsprojected on the curtain in case of a deformed curtain,

FIG. 9 d shows, by way of example, in a three dimensional view, aforeign object positioned between the monitoring unit and the curtain,

FIG. 10 a shows, by way of example, in a side view, a pattern projectedon the curtain,

FIG. 10 b shows, by way of example, in a side view, a pattern projectedon the curtain,

FIG. 10 c shows, by way of example, in a side view, a search portion,which contains several sub-patterns,

FIG. 10 d shows, by way of example, in a side view, a search portion,which contains several sub-patterns,

FIG. 10 e shows, by way of example, in a side view, a first sub-patternand a second sub-pattern,

FIG. 10 f shows, by way of example, in a side view, a search portion,which contains several sub-patterns,

FIG. 10 g shows, by way of example, an image of the pattern of FIG. 10a,

FIG. 11 a shows, by way of example, in a side view, a pointer lineprojected on the curtain,

FIG. 11 b shows, by way of example, an image of the pointer line in areference state,

FIG. 11 c shows, by way of example, an image of the pointer line in caseof a deformed curtain,

FIG. 12 shows, by way of example, a control system for the curtaincoating apparatus,

FIG. 13 shows, by way of example, in a side view, providing a spatialdistance distribution of the curtain,

FIG. 14 a shows, by way of example, a web having one or more defectportions,

FIG. 14 b shows, by way of example, defect data associated with a web

FIG. 14 c shows, by way of example, surveillance data associated with aweb, and

FIG. 15 shows, by way of example, in a three dimensional view,monitoring a moving web during curtain coating.

DETAILED DESCRIPTION

FIG. 1 shows a curtain coating apparatus 900, which may be arranged toproduce e.g. a coated web WEB1 or a laminate web WEB3. The apparatus 900may comprise a distributor unit 510, which may be arranged to form alayer CUR1, which comprises a coating material ADH0. The layer CUR1 maybe called e.g. as a curtain or veil. The material ADH0 of the curtainCUR1 may be substantially in a liquid state. The material ADH0 of thecurtain CUR1 may fall downwards towards a moving primary web WEB0 inorder to form a coating layer ADH1 on the primary web WEB0. A coated webWEB1 may be produced by coating the moving primary web WEB0 with thecoating layer ADH1. The coating layer ADH1 of the coated web WEB1 may beformed such that the material ADH0 of the curtain CUR1 at leasttemporarily adheres to the primary web WEB0.

The back surface of the curtain CUR1 may contact the primary web WEB0 ata contact line CL1. The primary web WEB0 may be moved at a velocity v₁with respect to a stationary reference REF0. The stationary referenceREF0 may be e.g. a predetermined point of the distributor unit 510. Thecoating layer ADH1 of the coated web WEB1 may comprise or consist ofcoating material ADH0, which covered the primary web WEB0 after fallingfrom the distributor unit 510.

The primary web WEB0 and/or the coated web WEB1 may be supported by asupporting member 550. The supporting member 550 may be e.g. a roll, aconveyer belt or an air cushion unit. The supporting member 550 may belocated beneath the contact line CL1. The primary web WEB0 and/or thecoated web WEB1 may be directly or undirectly moved by a roll. The webWEB0, WEB1 may be moved e.g. by one or more of the rolls 550, 552, 554.

The curtain CUR1 may be located between a first gas-liquid interface anda second gas-liquid interface such that the material ADH0 may freelyfall downwards e.g. due to gravity. The first gas-liquid interface maybe called e.g. as the front surface FS1 of the curtain CUR1, and thesecond gas-liquid interface may be called e.g. as the back surface BS1of the curtain CUR1 (FIG. 3 a). During normal operation, the frontsurface FS1 of the curtain CUR1 may be substantially vertical.

The apparatus 900 may comprise a monitoring unit 400, which may bearranged to monitor the curtain CUR1 during curtain coating. Theapparatus 900 may comprise a control system 901 for gatheringmeasurement data from the monitoring unit 400, for processing themeasurement data, and/or for controlling operation of the apparatus 900.In particular, the control system 901 may be arranged to controloperation of the apparatus 900 based on measurement data obtained fromthe monitoring unit 400.

The coated web WEB1 may be optionally coated with a second web WEB2 inorder to form a laminate web WEB3. The laminate web WEB3 may be producedby combining the coated web WEB1 with the second web WEB2 such that thecoating layer ADH1 remains between the primary web WEB0 and the secondweb WEB2. The second web WEB2 may be optionally pressed against thecoating layer ADH1 e.g. by compression rolls 552, 554.

The coating layer ADH1 may be optionally cured e.g. by heating and/or byusing ultraviolet radiation. The coating layer ADH1 may be cured e.g.before laminating the coated web WEB1 with the second web WEB2.

The primary web WEB0 may be provided e.g. by unwinding a roll RLL1, orby obtaining the primary web WEB0 from an additional web processingunit. The second web WEB2 may be provided e.g. by unwinding a roll RLL2,or by obtaining the second web WEB2 from an additional web processingunit. The laminate web WEB3 or the coated web WEB1 may be e.g. reeled upto form a roll RLL3. The laminate web WEB3 or the coated web WEB1 may beoptionally guided to a further processing step. The laminate web WEB3 orthe coated web WEB1 may be optionally cut to form a plurality of sheets.The coated web WEB1 or the laminate web WEB3 may be cut into sheets e.g.by using die-cutting or laser cutting. One or more portions may be cutfrom the coated web WEB1 or the laminate web WEB3 by e.g. by usingdie-cutting or laser cutting.

SX, SY and SZ denote orthogonal directions. The directions SX, SY and SZdefine a coordinate system fixed to the apparatus 900. The direction SYmay be anti-parallel with the direction of the gravity. The material ofthe curtain CUR1 may fall substantially in the direction of the gravity.The direction SX may be transverse to the direction of movement of theweb WEB0 in the vicinity of the contact line CL1. The direction SX maybe substantially perpendicular to the direction of movement of the webWEB0 in the vicinity of the contact line CL1. The contact line CL1 maybe aligned with the direction SX. The coated web WEB1 may be supportedby a roll 550, which has an axis AX1 of rotation, wherein the directionSX may be substantially parallel with the axis AX1 (FIG. 4). A positionin the coordinate system may be defined by coordinates x,y,z,respectively.

The primary web WEB0 may comprise e.g. paper or plastic. The second webWEB2 may comprise e.g. paper or plastic. The WEB0, WEB2 may comprisee.g. polyester and/or polypropylene.

The apparatus 900 may be arranged to produce e.g. label web WEB1, or alabel laminate web WEB3. The coating material ADH0 may comprise orconsist of e.g. adhesive material. The adhesive material ADH0 maycomprise e.g. polyurethane adhesive, and/or acrylic adhesive. Theadhesive material ADH0 may consist of polyurethane adhesive and/oracrylic adhesive. The coating layer ADH1 of the coated web WEB1 may bearranged to operate e.g. as a pressure sensitive adhesive layer, as aheat-activatable adhesive layer, as a heat-curable adhesive layer,and/or as a radiation curable adhesive layer. The webs WEB0, WEB1, WEB2,WEB3 may be flexible.

The primary web WEB0 may be used e.g. as the carrier layer of a labelweb or as the carrier layer of a label laminate web, and the second webWEB2 may be used e.g. as a release liner of the label laminate web. Thecarrier layer may also be called e.g. as a feedstock layer. The releaseliner WEB2 may comprise e.g. plastic, paper or cardboard coated with ananti-adhesion agent. The release liner WEB2 may comprise ananti-adhesion layer, which comprises anti-adhesion agent. Theanti-adhesion agent may comprise e.g. silicone and/or a fluoropolymer.One or more adhesive labels LABEL1 may be produced by cutting from theweb WEB3, and by removing the release liner.

Alternatively, the primary web WEB0 may be used as a release liner of alabel laminate web, and the second web WEB2 may be used e.g. as thecarrier layer of the label laminate web. The release liner WEB2 maycomprise an anti-adhesion layer, which comprises anti-adhesion agent.The anti-adhesion agent may comprise e.g. silicone and/or afluoropolymer. The primary web WEB0 may comprise e.g. plastic, paper orcardboard coated with an anti-adhesion agent. One or more adhesivelabels LABEL1 may be produced by cutting from the web WEB3, and byremoving the release liner.

The thickness of the web WEB0, WEB1, WEB2 and/or WEB3 may be e.g. in therange of 0.01 mm to 0.5 mm, in particular in the range of 0.02 mm to 0.3mm. In an embodiment, the WEB0, WEB1, WEB2 and/or WEB3 may betransparent or translucent.

The coating material ADH0 may also comprise or consist of ananti-adhesion agent. The anti-adhesion agent may comprise e.g. siliconeand/or a fluoropolymer. The coating material ADH0 does not need tocomprise an adhesive.

FIG. 2 a shows a label laminate web WEB3, which may be obtained bycurtain coating. The label laminate web WEB3 may comprise a carrierlayer WEB0, an adhesive layer ADH1, and a release liner WEB2. The labellaminate web WEB3 may comprise one or more visually detectable markingsINFO1 produced in and/or on the carrier layer WEB0. The marking INFO1may comprise e.g. text, a trade mark pattern, or a barcode.

The directions SX′, SY′ and SZ′ define a coordinate system of the coatedweb WEB1, when the WEB1 is straightened so that it is substantiallyplanar. The directions SX′, SY′ and SZ′ are orthogonal. The directionSY′ may denote the machine direction of the coated web WEB1, and SX′ maydenote the cross machine direction of the coated web WEB1. The term“machine direction” SY″ means that the primary web WEB0 moved in thedirection SY′ in the vicinity of the contact line CL1 when the coatingmaterial ADH0 was brought into contact with the primary web WEB0.

FIG. 2 b shows a label LABEL1 attached to an item ITE1. The item ITE1may be e.g. container. The container may be e.g. a glass bottle, aplastic bottle or a cardboard box. The label LABEL1 attached to an itemITE1 e.b. by cutting a piece from the label laminate web WEB3 of FIG. 2a, peeling the release liner WEB2 away from the LABEL1, and bringing theLABEL1 into contact with the item ITE1 so that the LABEL1 sticks to thesurface of the item ITE1. The label may be optionally pressed and/orheated in order to improve adhesion. The adhesive layer ADH1 maycomprise adhesive ADH0, which may be e.g. pressure sensitive, thermallyactivatable, or UV curable.

In an embodiment, the adhesive layer ADH1 of the label web WEB2 does notneed to be covered with a release liner. The label web may be storedand/or transported as a linerless label web WEB1. The linerless labelmay be activated e.g. by heat prior to attaching it to an item ITE1.

FIG. 2 c shows a label laminate web WEB3, which may be obtained bycurtain coating. The label laminate web WEB3 may comprise a carrierlayer WEB2, an adhesive layer ADH1, and a release liner WEB0. The labellaminate web WEB3 may comprise one or more visually detectable markingsINFO1 produced in and/or on the carrier layer WEB0. The marking INFO1may comprise e.g. text, a trade mark pattern, or a barcode.

FIG. 2 d shows a label LABEL1 attached to an item ITE1. The item ITE1may be e.g. container. The container may be e.g. a glass bottle, aplastic bottle or a cardboard box. The label LABEL1 attached to an itemITE1 e.b. by cutting a piece from the label laminate web WEB3 of FIG. 2c, peeling the release liner WEB0 away from the LABEL1, and bringing theLABEL1 into contact with the item ITE1 so that the LABEL1 sticks to thesurface of the item ITE1. The label may be optionally pressed and/orheated in order to improve adhesion. The adhesive layer ADH1 maycomprise adhesive ADH0, which may be e.g. pressure sensitive, thermallyactivatable, or UV curable.

FIG. 3 a shows the curtain CUR1 and the coated web WEB1 in across-sectional side view. The coating material ADH0 of the curtain CUR1may substantially continuously fall downwards e.g. due to gravity, dueto a pulling force created by the moving web WEB0, and/or due to inertiaof the coating medium ADH0 discharged from the distributing unit 510.The primary web WEB0 may be moved in a direction, which is transverse tothe direction of gravity. The back surface BS1 of the curtain CUR1 maycontact the primary web WEB0 at a contact line CL1. The coatingapparatus 900 may be arranged to operate e.g. such that the contact lineCL1 is substantially straight. However, the shape of the curtain CUR1may momentarily deviate from the substantially planar form e.g. due toinstable flow of the material ADH0 through the distributor unit 510, dueto fluctuations caused by a fluctuating pressure difference p₁−p₀ overthe curtain CUR1 and/or due to fluctuations caused by a pulling(cohesive) force created by the moving coating layer ADH1. Thus, thecontact line CL1 may be straight or curved depending on the momentaryshape of the curtain CUR1.

z_(CL1)(x,t) denotes a horizontal distance between contact line CL1 anda reference plane REF00 at a lateral position x and at the time t. Thecontact line CL1 may be substantially parallel to the direction SX (FIG.4). The distributor unit 510 may comprise e.g. shelf structure and/or anozzle for forming the curtain CUR1. The curtain CUR1 may have aninitial thickness d₀(x) at the lateral position x. The material ADH0 ofthe curtain CUR1 may fall at each point (x,y,z) a velocity v_(C)(x,y,z).The front surface FS1 of the curtain CUR1 may have a height h_(C). Thecurtain CUR1 may have an instantanous thickness d(x,y,t) at eachposition (x,y) at a time t.

The primary web WEB0 and the coated web WEB1 may be supported e.g. bythe roll 550. v₁ denotes the velocity of the primary web WEB1 at thecontact line CL1. v₁ denotes the velocity of the uncovered primary webWEB0 with respect to a stationary reference REF0. The velocity v₁ of theprimary web WEB0 may be e.g. in the range of 0.5 m/s to 60 m/s. Thevelocity v₁ of the primary web WEB0 may be e.g. in the range of 5 m/s to40 m/s. The velocity v₁ of the primary web WEB0 may be e.g. in the rangeof 0.5 m/s to 5 m/s. Using a lower velocity v₁ may improve stability ofthe curtain CUR1.

The roll 550 may be rotated at an angular velocity ω₁ so that theprimary web WEB0 may move at the velocity v₁. The velocity of the coatedweb WEB1 may be substantially equal to the velocity v₁ of the primaryweb WEB0. The curtain coating may be performed substantially withoutstretching the primary web WEB0. If the method comprises forming alaminate web WEB3, the temporally averaged velocity of the laminate webWEB3 may be substantially equal to the temporally averaged velocity v₁of the primary web WEB0.

The moving surface of the primary web WEB0 may create a moving layer ofgas GAS0. The moving layer of gas GAS0 may be called e.g. as a boundarylayer GAS0. The gas flow GAS0 may potentially cause deformation of thecurtain CUR1. The gas flow GAS0 may potentially cause random and/orperiodic oscillation of the curtain CUR1, which in turn may cause aspatially non-uniform thickness d₁ of the coating ADH1. The boundarylayer GAS0 may cause e.g. formation of gas bubbles between the primaryweb WEB0 and the coating layer ADH1. The apparatus 900 may optionallycomprise a gas flow stabilizing unit 520 for controlling the pressuredifference p₁−p₀ over the curtain CUR1. p₁ denotes local pressure at thefront surface FS1 of the curtain CUR1. p₀ denotes local pressure at theback surface BS1 of the curtain CUR1. The pressure difference p₁−p₀ maycause deflection and/or deformation of the curtain CUR1. The stabilizingunit 520 may reduce or control the gas flow GAS0. A gas flow GAS1 of thestabilizing unit 520 may be arranged to minimize or prevent formation ofthe air bubbles. In an embodiment, the stabilizing unit 520 may bearranged to cause a secondary air flow GAS2 e.g. in order to control thepressure p₀ between the curtain CUR1 and the stabilizing unit 520. Themarking GAS2 may also denote gas, which is in contact with the curtainCUR1 and with the primary WEB0.

The curtain CUR1 may be optically monitored by a monitoring unit 400.The monitoring unit 400 may be arranged to monitor the deflection and/ordeformation of the curtain CUR1. The monitoring unit 400 may be arrangedto detect deflection and/or deformation of the curtain CUR1. Themonitoring unit 400 may be arranged to measure a distance between themonitoring unit 400 and the curtain CUR1, and the monitoring unit 400may also be called e.g. as a distance sensor unit. The monitoring unit400 may be arranged to measure deflection and/or deformation of thecurtain CUR1. The monitoring unit 400 may be arranged to measure aninstantaneous three-dimensional shape of the curtain CUR1. Themonitoring unit 400 may be arranged to measure an instantaneousthree-dimensional shape of the front surface FS1 of the curtain CUR1.The monitoring unit 400 may be arranged to detect whether the curtainhas one or more voids.

The stabilizing unit 520 may have a front surface 521. In an embodimenta predetermined point of the stabilizing unit 520 may be used as areference point REF0. The reference point REF0 may be in the field ofview of the monitoring unit 400. In an embodiment, the front surface 521may be substantially planar, and the front surface 521 may define thereference plane REF00. A part of the front surface 521 may be in thefield of view of the monitoring unit 400 e.g. when the curtain is absentand/or when the monitoring unit 400 detects the front surface 521through a void.

Referring to FIG. 3 b, the curtain CUR1 may meet the primary web WEB0 atthe contact line CL1. The contact line CL1 may be defined by the primaryweb WEB0 and the curtain CUR1. The coating material ADH0 of the curtainCUR1 may be brought into contact with the primary web WEB0 at thecontact line CL1. The back side BS1 of the curtain CUR1 may represent agas-liquid interface between a gas GAS2 and the coating material ADH0.The position of the contact line CL1 may be defined by the gas GAS2, bythe primary web WEB0, and by the coating material ADH0 of the curtainCUR1. Three different phases may meet at the contact line CL1. The gasGAS0, the surface of the primary web WEB0, and the coating material ADH0may meet at the contact line CL1. The coating material ADH0 of thecurtain CUR1 may move substantially downwards with respect to thecontact line CL1. The primary web WEB0 may move with respect to thecontact line CL1. The velocity of the primary web WEB0 with respect tothe contact line CL1 may be substantially equal to the velocity v₁ ofthe primary web WEB0 with respect to a stationary reference. However,the velocity of the primary web WEB0 with respect to the contact lineCL1 may temporarily be slightly different from the velocity v₁, becausethe position of the contact line CL1 may move according to thedeflections and/or deformations of the curtain CUR1.

FIGS. 1, 3 a and 3 b illustrate some examples of curtain coatingarrangements, some effects related to the behavior of the curtain flow,and some parameters related to the behavior of the curtain flow. Thecoating arrangements may be varied e.g. depending on the properties ofthe primary web WEB0 and/or depending on the properties of the coatingmaterial ADH0.

FIG. 4 shows, in a three dimensional view, the coating apparatus 900.The primary web WEB0 may be supported e.g. by a roll 550. The contactline CL1 may be substantially parallel to an axis AX1 of rotation of theroll 550. The width w₁ of the coating layer ADH1 of the coated web WEB1may be e.g. in the range of 0.5 m to 4.0 m. The curtain CUR1 may have awidth w_(C). The width w_(C) may be substantially equal to the width w₁.The web WEB0, WEB1 may have a width W_(WEB). The width w_(WEB) may begreater than or equal to the width w₁. The width w_(WEB) may be e.g. inthe range of 0.5 m to 4.0 m. The width w₁ of the coating layer ADH1 ofthe coated web WEB1 may be e.g. in the range of 0.5 m to 15 m. The widthw_(WEB) of the web may be e.g. in the range of 0.5 m to 15 m.

Referring to FIG. 5 a, the curtain CUR1 may momentarily have one or morevoid portions VOID1, VOID2, which do not comprise the coating materialADH0, or where the thickness d(x,y,t) of the curtain is abnormally low.The void portions VOID1, VOID2 may be called e.g. as voids or holes.

The curtain CUR1 may have a normal boundary BND1. The normal boundaryBND1 may be defined by the perimeter of the curtain CUR1 during normaloperation. A void portion VOID1, VOID2 may be a portion which is locatedwithin the normal boundary BND1, but which does not comprise the coatingmaterial ADH0.

The curtain CUR1 may have a normal thickness d_(REF)(x,y) at eachlocation (x,y) within the normal boundary BND1. The thicknessd_(REF)(x,y) does not need to be spatially constant. The curtain CUR1may have an instantaneous thickness d(x,y,t) at each location (x,y)within the normal boundary BND1. The curtain CUR1 may momentarily haveone or more thin portions where the instantaneous thickness d(x,y,t) issubstantially lower than the normal thickness d_(REF)(x,y) of thecurtain CUR1. In an embodiment, the term “void portion” may also referto a thin portion. The term “void portion” may refer to a portion, wherethe instantaneous thickness d(x,y,t) of the curtain CUR1 issubstantially lower than the normal thickness d_(REF)(x,y) of thecurtain CUR1.

The voids VOID1, VOID2 may be caused e.g. by the instability of thecurtain CUR1, due to a locally blocked distributor unit 510 and/or dueto wrong operating parameters of the coating apparatus 900. The coatedweb WEB1 may comprise one or more defective portions F_(j), F_(j+1) (seeFIG. 14 a) where the coating material ADH0 may be missing or where thethickness d₁ of the coating layer ADH1 may be abnormally low. Thelateral positions of the defect portions F_(j), F_(j+1) may correspondto the lateral positions of the voids VOID1, VOID2. The lateralpositions of the defect portions F_(j), F_(j+1) may be determined bymonitoring the lateral positions of the voids VOID1, VOID2. Thelongitudinal positions of the defect portions F_(j), F_(j+1) may bedetermined by monitoring with sufficient temporal resolution when thevoids VOID1, VOID2 are present, and by using the known velocity v₁ ofthe primary web WEB0.

The curtain CUR1 may comprise one or more continuous portions POR1, POR2where the thickness of the curtain is substantially equal to a normalvalue. The curtain CUR1 may comprise one or more continuous portionsPOR1, POR2 where the thickness of the curtain is greater than or equalto a threshold value.

The presence of the voids VOID1, VOID2 may be monitored e.g. byilluminating the curtain CUR1 with illuminating light LB3, and bycapturing one or more images FRAME2 of the curtain by an imaging unit300. The illuminating light LB3 may be provided by an illuminating unit600.

The illuminating unit 600 may comprise e.g. a fluorescent lamp, one ormore light emitting diodes (i.e. LEDs), one or more lasers and/or one ormore gas discharge lamps (e.g. a xenon flash lamp). The illuminatingunit 600 may comprise a light-emitting surface whose width w₆₀₀ isgreater than or equal to 50% of the width w₁ of the coating layer ADH1of the coated web WEB1. The illuminating light LB3 may be e.g. visiblelight. In an embodiment, at least 50% of the optical power of the lightLB3 may be in the wavelength range of 400 nm to 760 nm.

The illuminating unit 600 may be arranged to illuminate the curtain CUR1with the illuminating light LB3 such that the spatial intensitydistribution of the light LB3 at the front surface of the curtain CUR1is substantially uniform. The width W₆₀₀ may be greater than or equal to100% of the width w₁ of the coating layer ADH1 of the coated web WEB1.The illuminating unit 600 may comprise a light emitting surface, whereinthe width W₆₀₀ of the light emitting surface may be greater than orequal to the width w_(C) of the curtain CUR1, and the transversedimension of the light emitting surface may be greater than or equal tothe height h_(C) of the curtain CUR1. The transverse dimension may bedefined in a direction perpendicular to the direction SX. The lightemitting surface may be implemented e.g. by a transmissive or reflectiveoptical diffuser and/or by using a plurality of light sources arrangedin a one-dimensional or two-dimensional array. The illuminating lightLB3 may be pulsed or continuous. In case of pulsed light, the operationof the imaging unit 300 may be syncronized with the pulses of theilluminating light LB3.

In an embodiment, the illuminating unit 600 may illuminate the curtainCUR1 also in a non-uniform but temporally stable manner. In this case,the threshold values for detecting a void may depend on the position(x,y). The threshold values may be stored in a memory e.g. as pixelvalues of a reference image. In this case, the reference image may becalled e.g. as a normalization image or as a threshold value image. Thenormalization image may be determined e.g. by temporarily placing alight-scattering (dummy) sheet to the position of the curtain CUR1,illuminating the dummy sheet with the (non-uniform) illumination, and bycapturing a digital image of the sheet. During normal operation, thepixel values of a captured image FRAME2 may be optionally normalized byusing the normalization image.

The layer CUR1 may provide reflected light LB4 by reflecting theilluminating light LB3. The refelcted light LB4 may be provided e.g. byscattering from the material of the layer CUR1, by specular reflectionfrom the surface FS1, and/or by specular reflection from the surfaceBS1.

Referring to FIG. 5 b, the monitoring unit 400 may comprise an imagingunit 300, which may be arranged to detect the presence of voids VOID1,VOID2. The voids VOID1, VOID2 may be detected by analyzing an imageFRAME2 captured by the imaging unit 300, e.g. by using patternrecognition. The imaging unit 300 may comprise imaging optics 320 and animage sensor 310. The imaging optics 320 may be arranged to form anoptical image IMG2 on the image sensor 320 by focusing light LB3reflected from the curtain CUR1. The image sensor 320 may be arranged toconvert the optical image IMG2 into a digital image FRAME2. The digitalimage FRAME2 may also be called e.g. as an image frame.

In an embodiment, the imaging unit 300 may be selectively sensitive tovisible light. The imaging unit 300 may be arranged to operate such thatit is substantially insensitive to spectral components at wavelengthslonger than 760 nm.

The image sensor 310 may comprise a two-dimensional array of detectorpixels. The image sensor 310 may be e.g. CMOS image sensor or a CCDimage sensor. The image sensor 310 may be e.g. a monochrome image sensoror RGB image sensor. The imaging unit 300 may be sensitive to visiblelight LB4.

The imaging unit 300 may have a field of view VIEW2. x_(F) may denotethe lateral position of the void VOID1 with respect to a locationreference REF0. Δx_(F) may denote the width of the void VOID1 whendetermined on the surface of the primary web WEB0.

The monitoring unit 400 may further comprise a projecting unit 200 and asecond imaging unit 100 for measuring distances by triangulation.

FIG. 5 c shows an image frame FRAME2 provided by the imaging unit 300.The image frame FRAME2 may be a digital image corresponding to anoptical image IMG2 formed at a time t₂. The image frame FRAME2 maycomprise an image SCUR1 of the curtain CUR1. The image frame FRAME2 maycomprise an image SPOR1 of a first continuous portion POR1 of thecurtain CUR1. The image frame FRAME2 may comprise an image SPOR2 of asecond continuous portion POR2 of the curtain CUR1. The image frameFRAME2 may comprise an image SVOID1 of the void portion VOID1. The imageframe FRAME2 may comprise a point SREF0, which corresponds to thereference position REF0.

The images SCUR1, SPOR1, SPOR2, SVOID1 may also be called e.g. assub-images. The presence of the image SVOID1 of the void portion VOID1may be determined e.g. by using an image recognition algorithm. Theboundaries of the image SVOID1 of the void portion VOID1 may bedetermined e.g. by using an image recognition algorithm. The boundariesof the image SVOID1 of the void portion VOID1 may be determined e.g. byusing signal level thresholding. The digital image FRAME2 may comprise aplurality of image pixels such that each image pixel has a signal value.If the signal value of a pixel is lower (or higher) than a predeterminedthreshold value, said pixel may be determined to overlap the imageSVOID1 of a void portion VOID1. The position u(t₂) of the sub-imageSVOID1 with respect to the reference position ORIG2 may be determined byanalyzing the digital image FRAME2. The reference position ORIG2 may bee.g. at a predetermined corner of the image FRAME2 or at the pointSREF0. The width Δu(t₂) of the sub-image SVOID1 may be determined byanalyzing the digital image FRAME2. u_(max) may denote the total widthof the digital image FRAME2. The width u_(max) may be expressed e.g. asa number of pixels. u_(max) may be e.g. in the range of 100 pixels to10000 pixels.

The values of u(t₂) and Δu(t₂) may be expressed e.g. as a number ofpixels (for example u(t₂) may be equal to 100 pixels) or as relativevalues (for example u(t₂) may be equal to 55% of u_(max)).

The directions SU and SV may define a coordinate system of the imageframe FRAME2. The direction SU may correspond to the direction SX of thereal space (i.e. the direction SU may a be an image of the directionSX). The direction SV may correspond to the direction SY of the realspace.

The position of the void VOID1 may be determined based on the positionof the sub-image SVOID1. The lateral position x′_(j) of a defect portionF_(j) (see FIG. 14 a) caused by the void VOID1 may be determined basedon the position u(t₂) of the sub-image SVOID1 of the void VOID1.

The image frame FRAME2 may be optionally stored in a memory. The imageframe FRAME2 may be used at a later stage e.g. as a documentation image.

Referring to FIG. 6, a monitoring unit 400 may be arranged to providedistance information. The monitoring unit 400 may be arranged to measuredistances to one or more pointer features D_(A,1), D_(A,2), D_(A,k),D_(A,k+1), D_(A,m) by triangulation. The monitoring unit 400 maycomprise a projecting unit 200, which may be arranged to project apattern PTRN1 on the curtain CUR1. The projected pattern PTRN1 maycomprise e.g. one or more pointer features D_(A,1), D_(A,2), D_(A,k),D_(A,m). The pointer features D_(A,1), D_(A,2), D_(A,k), D_(A,m) may bee.g. substantially circular spots, elliptical spots, or line sections.In an embodiment, the spots may be dots.

The pointer features may be projected on the curtain CUR1 by providingilluminating light beams LB1, which impinge on the curtain CUR1. Inother words, the curtain CUR1 may operate as a display screen, whichconsists of a falling liquid film. The monitoring unit 400 may bearranged to receive light LB2 reflected from the curtain CUR1. Thereflected light LB2 may be provided e.g. by scattering, which takesplace within the material ADH0 of the curtain CUR1. For example, thematerial ADH0 may comprise particles, which scatter light. The materialADH0 may comprise a heterogeneous mixture, which is arranged to scatterlight.

In particular, the material ADH0 may be selected such that the materialADH0 scatters light at the wavelength λ₁ of the projected light beamsLB1. The wavelength λ₁ may be e.g. in the infrared region. Thewavelength λ₁ may be e.g. in the range of 780 nm to 3000 nm. Thematerial ADH0 may comprise light-scattering particles. The material ADH0may be e.g. an adhesive composition, which comprises e.g. polyestersegments and/or filler material particles to enhance scattering.

In an embodiment, the directions of the illuminating light beams LB1 maybe selected such that the reflected light LB2 may be provided byspecular reflection, which takes place at the front surface FS1 of thecurtain CUR1. The reflected light LB2 may also be provided by specularreflection, which takes place at the back surface BS1 of the curtainCUR1.

The position of each pointer feature may be defined by coordinates x andy. For example, the position of the pointer feature D_(A,k) may bedefined by the coordinates x_(k) and y_(k).

Referring to FIG. 7 a, the monitoring unit 400 may comprise a projectingunit 200, which may be arranged to project one or more illuminatinglight beams LB1₁, LB1₂, LB1_(k), LB1_(k+1), LB1_(m) such that the beamsmay impinge on the curtain CUR1 at the locations of the featuresD_(A,1), D_(A,2), D_(A,k), D_(A,k+1), D_(A,m). The light LB1 maycomprise one or more beams LB1₁, LB1₂, LB1_(k), LB1_(k+1), LB1_(m). Thecurtain CUR1 may be arranged to provide reflected light LB2 byreflecting and/or scattering light of the light beams LB1. The pointerfeatures D_(A,1), D_(A,2), D_(A,k), D_(A,k+1), D_(A,m) are formed by thereflecting and/or scattering, which takes place at the locations of saidfeatures. The monitoring unit 400 may be arranged to measure theposition of one or more pointer features D_(A,1), D_(A,2), D_(A,k),D_(A,k+1), D_(A,m) by receiving light LB2 reflected from the curtainCUR1. The position of a pointer feature D_(A,k) may be expressed e.g. bya horizontal coordinate x_(k) and by a vertical coordinate y_(k). Thecoordinates x_(k) and y_(k) may define the position of the pointerfeature D_(A,k) e.g. with respect to a reference point REF0. Thereference point REF0 may be e.g. at a prominent feature of the apparatus900, e.g. at a predetermined point of the distributor unit 510 or at apredetermined point of the stabilizing unit 520.

The monitoring unit 400 may comprise an imaging unit 100, which may havea field of view VIEW1 and a viewing sector SEC1. L₀ may denote adistance between the projecting unit 200 and the imaging unit 100 of themonitoring unit 400. L₁ may denote a distance between the principalpoint PP1 of the imaging unit 100 and a reference plane, which includesthe reference point REF0. The reference plane may be defined e.g. by thedirections SX, SY. In particular, L₁ may denote a distance between theprincipal point PP1 of the imaging unit 100 and the surface 521 of thegas flow stabilizing unit 520 (see FIG. 3 a). The width of the field ofview VIEW1 at the location of the curtain CUR1 may be e.g. greater thanor equal to the width w1 of the coating layer ADH1 of the coated webWEB1. The three-dimensional shape of the curtain may be defined e.g. bya shape function z(t,x,y). The shape function z(t,x,y) may betime-dependent. The shape function z(t,x,y) may define the position ofthe curtain CUR1 in the direction SZ as the function of time t and asthe function of the coordinates x and y. The monitoring unit 400 may bearranged to measure the shape of the curtain CUR1. The monitoring unit400 may be arranged to measure the instantaneous shape of the curtainCUR1 substantially in real time. L_(NORM) may denote a normal distancebetween the layer CUR1 and the principal point PP1 of the imaging unit100.

FIG. 7 b illustrates capturing an image SD_(A,k) of a pointer featureD_(A,k) in a reference situation prevailing at a time t₀. The referencesituation may refer e.g. to a situation where the curtain CUR1 issubstantially planar. The reference situation may refer e.g. to asituation where the curtain CUR1 has a temporally stable shape.

The projecting unit 200 may comprise e.g. a light source 210 arranged toprovide a primary beam LB0, and an optical element 220 arranged toprovide one or more illuminating light beams LB1 by deflecting light ofthe primary beam LB0. The deflection may comprise deflection bydiffraction, deflection by refraction and/or deflection by reflection.

In particular, the light source 210 may comprise e.g. a laser arrangedto provide a laser beam LB0. The optical element 220 may be e.g. aholographic element or a diffraction grating, which may be arranged toprovide several illuminating light beams LB1 by diffracting light of thelaser beam LB0 such that the light beams LB1 may simultaneouslypropagate in different directions. One of the light beams LB1 may bemarked e.g. with the symbol LB1_(k). The curtain CUR1 may form thepointer feature D_(A,k) by reflecting and/or scattering light of thebeam LB1_(k). Thus, the pointer feature D_(A,k) may be formed at thelocation where the beam LB1_(k) impinges on the curtain CUR1. Thematerial ADH0 of the curtain CUR1 may provide reflected light LB2_(k) byreflecting and/or scattering light of the beam LB1_(k). The lightLB2_(k) may be provided by specular reflection and/or by scattering.Scattering may cause diffuse reflection and/or diffuse refraction.

The imaging unit 100 may be arranged to form an optical image IMG1 byreceiving the reflected light LB2_(k). The imaging unit 100 may compriseimaging optics 120, which may be arranged to form an image SD_(A,k) ofthe feature D_(A,k) on an image sensor 110 by focusing reflected lightLB2_(k). The image SD_(A,k) may substantially coincide e.g. with adetector pixel DP_(q) at the time t₀. The image sensor 110 may convertthe optical image IMG1 into a digital image FRAME0 (FIG. 8 b), or into adigital image FRAME1 (FIG. 8 c). The imaging optics 120 and the imagesensor 110 may be selected such that the imaging unit 100 may besensitive to the wavelength λ1 (or wavelength band) of the illuminatinglight LB1. The light of the illuminating light LB1 may be e.g. in theinfrared range. For example, at least 90% of the optical power of thelight LB1 may be at wavelengths longer than 760 nm.

The pointer feature D_(A,k) may be formed by an illuminating light beamLB1_(k). The direction of the light beam LB1_(k) may be defined e.g. byan angle α_(k). The angle α_(k) may be e.g. an angle between thecenterline of the light beam LB1_(k) and the direction SX.

The imaging optics 120 of the imaging unit 100 may have a principalpoint PP1. VLIN_(k) denotes a line defined by the principal point PP1and the pointer feature D_(A,k). The line VLIN_(k) may be called e.g. asa line of sight. The direction of the line VLIN_(k) may be defined e.g.by an angle β_(k). The angle β_(k) may be e.g. an angle between the lineVLIN_(k) and the direction SX. The angle β_(k) may be determined e.g.from the position of the image SD_(A,k) formed on the image sensor 110.The angle β_(k) may be determined e.g. from the position of the detectorpixel DP_(q). The distance L_(k)(t₀) between the feature D_(A,k) and theprincipal point PP1 may be determined from the angle α_(k), from theangle 3 _(k), and from the base distance L_(o). The distance L_(k)(t₀)between the feature D_(A,k) and the principal point PP1 may bedetermined by triangulation from the angle α_(k), from the angle β_(k),and from the base distance L₀.

Referring to FIG. 7 c, a portion DEFPOR1 of the curtain CUR1 may bemomentarily deformed and/or displaced at a time t₁. L_(k)(t₁) may denotethe distance between the principal point PP1 and the pointer featureD_(A,k) at the time t₁. The distance L_(k)(t₁) at the time t₁ may bedifferent from the distance L_(k)(t₀) at the time t₀ due to thedeformation/displacement of the curtain CUR1. The position of the imageSD_(A,k) of the feature D_(A,k) on the image sensor may depend on thedistance between the principal point PP1 and the pointer featureD_(A,k). The image SD_(A,k) may substantially coincide e.g. with adetector pixel DP_(q′) at the time t₁.

The coordinate z_(k)(t₁,x_(k),y_(k)) of the curtain CUR1 at the positionof the feature D_(A,k) at the time t₁ may be determined by triangulationfrom the angle α_(k), from the angle β_(k), from the base distanceL_(o), and from the known position of the reference point REF0. Thecoordinate z_(k)(t₁,x_(k),y_(k)) may define the position of the curtainCUR1 in the direction SZ.

The image sensor 110 may comprise a two-dimensional array of detectorpixels. The image sensor 110 may be e.g. CMOS image sensor or a CCDimage sensor. The imaging unit 100 may be sensitive to infrared lightLB2. The image sensor 110 of the imaging unit 100 may be e.g. a CCDsensor or a CMOS sensor. CCD is an abbreviation for charge coupleddevice. CMOS is an abbreviation for Complementarymetal-oxide-semiconductor. The horizontal viewing sector SEC1 of theimaging unit 100 may be e.g. in the range of 30° to 60°.

The imaging unit 100 may be arranged to capture images FRAME1 at a rate,which may be e.g. in the range of 10 to 200 image frames per second, inorder to provide sufficient temporal resolution. The imaging unit 100may be arranged to capture image images FRAME1 at a rate, which may bee.g. in the range of 20 to 40 frames per second.

Pixel values of the image FRAME1 may be quantized e.g. by a localthresholding operation to provide a binary image. The value of eachpixel of the binary image may be either 0 or 1. The binary image may besubsequently analyzed at a high rate by a data processor e.g. fordetermining the identity for each sub-image SG_(k), and/or for neighborextraction.

The monitoring unit 400 may be arranged to provide distance information.The distance information may comprise e.g. one or more distance valuesL_(k)(t₁) and/or one or more coordinate values z_(k)(t₁,x_(k),y_(k)).The distance L_(k)(t₁) or depth information z_(k)(t₁,x_(k),y_(k)) may bedetermined at a resolution, which may be e.g. in the range of 0.1 mm to0.5 mm, 0.5 mm to 1 mm, 1 mm to 2 mm, or 2 mm to 4 mm. The resolutionmay depend on the distance between the curtain CUR1 and the principalpoint PP1, and on the length L_(o) of the baseline BL0. The resolutionmay depend on the angle γ_(k) between the direction of an illuminatinglight beam LB1_(k) and a line of sight VLIN_(k). The distance L_(NORM)between the curtain CUR1 and the principal point PP1 may be e.g. in therange of 0.5 to 4 m. In particular, the distance between the curtainCUR1 and the principal point PP1 may be e.g. in the range of 1.0 to 4.0m in order to minimize the risk of contaminating the optics with thecoating material and/or in order to provide space for maintenanceoperations. The distance L_(NORM) may be e.g. in the range of 0.5 m to3.0 m. The distance L_(NORM) may be e.g. in the range of 1.5 m to 3.0 m.The length of the baseline BL0 may be e.g. greater than or equal to 10mm in order to provide sufficient resolution for measuring the distanceL_(k)(t₁). The angle γ_(k) may be e.g. greater than or equal to 0.2degrees in order to provide sufficient resolution for measuring thedistance L_(k)(t₁).

The image FRAME1 may be slightly blurred e.g. due to defocusing and/oroptical aberrations. In an embodiment, the size of the pointer featuresmay be rather large in order to facilitate reliable detection of thepointer features by using the imaging unit 100. For example, thesmallest dimension (e.g. height, width or diameter) of the pointerfeatures D_(A,k) may be e.g. greater than 0.001 times the width w_(e) ofthe curtain CUR1.

The image sensor 110 may be arranged to provide a further image frameFRAME1B at a time t₃. The further image frame FRAME1B may be utilizede.g. for measuring distance information at the time t₃. In particular,the further image frame FRAME1B may be utilized e.g. for detecting thepresence of a foreign object O1 (FIG. 8 d).

FIG. 7 d shows projecting the primary pattern PTRN1 on the curtain CUR1.The primary pattern PTRN1 may comprise pointer features D_(A,1),D_(A,2), D_(A,k), D_(A,m). The curtain CUR1 may have a deformed portionDEFPOR1. The three-dimensional shape of the curtain CUR1 has beenvisualized in this example by an imaginary mesh pattern VIZ. It shouldbe understood that the pattern PTRN1 does not need to comprise the meshpattern VIZ.

FIG. 8 a shows a plurality of pointer features D_(A,1), D_(A,2),D_(A,k), D_(A,m) projected on the curtain CUR1. The pointer featureD_(A,k) may have a dimension h_(k). The dimension h_(k) may be e.g. theheight or a diameter of a dot.

FIG. 8 b shows a digital image FRAME1, which comprises images SD_(A,1),SD_(A,2), SD_(A,k), SD_(A,m) of the pointer features D_(A,1), D_(A,2),D_(A,k), D_(A,m). The image SD_(A,k) may be an image of the pointerfeature D_(A,k) The digital image FRAME1 may optionally comprise animage SCUR1 of the curtain CUR1, but this is not necessary. The imagesSD_(A,1), SD_(A,2), SD_(A,k), SD_(A,m), SCUR1 may be called assub-images. The digital image FRAME0 may represent the situation at atime t₀. u_(k)(t₀) denotes a position coordinate of the sub-imageSD_(A,k) in the image FRAME0. The position u_(k)(t₀) of the sub-imageSD_(A,k) representing the time t₀ may be determined by analyzing thedigital image FRAME0. The angle β_(k)(t₀) shown in FIG. 7 b may bedetermined from the position of the sub-image SD_(A,k), by analyzing thedigital image FRAME0. u_(max) may denote the total width of the digitalimage FRAME0.

FIG. 8 c shows a digital image FRAME1, which comprises the sub-imagesSD_(A,1), SD_(A,2), SD_(A,k), SD_(A,m) representing the time t₁, i.e.when the curtain CUR1 was displaced and/or deformed. u_(k)(t₁) denotes aposition of the sub-image SD_(A,k) in the image FRAME1 which wascaptured at the coating time t₁.

The position u_(k)(t₁) of the sub-image SD_(A,k) representing the timet₁ may be determined by analyzing the digital image FRAME1. The angleβ_(k)(t₁) shown in FIG. 7 c may be determined from the position of thesub-image SD_(A,k), by analyzing the digital image FRAME1. u_(max) maydenote the total width of the digital image FRAME1.

REF1_(k) denotes the position of the sub-image SD_(A,k) in the referenceimage FRAME0, i.e. at the coating time t₀. Δu_(k) denotes a displacementbetween the position of the sub-image SD_(A,k) at the time t₁ and theposition of the sub-image SD_(A,k) at the time t₀.

The position of a predetermined portion of the curtain CUR1 may bemonitored by:

-   -   projecting one or more pointer features D_(A,1), D_(A,2),        D_(A,k), D_(A,m) on the curtain CUR1,    -   capturing a digital image FRAME1 which comprises a sub-image        SD_(A,k) of a pointer feature D_(A,k), and    -   determining the position of the pointer feature D_(A,k) from the        position of the sub-image SD_(A,k) by triangulation.

The position of a predetermined portion of the curtain CUR1 may bemeasured by:

-   -   projecting one or more pointer features D_(A,1), D_(A,2),        D_(A,k), D_(A,m) on the curtain CUR1,    -   capturing a digital image FRAME1, which comprises a sub-image        SD_(A,k) of a pointer feature D_(A,k), and    -   determining the position of the pointer feature D_(A,k) from the        position of the sub-image SD_(A,k) by triangulation.

The deformation and/or displacement of the curtain may be measured by:

-   -   projecting one or more pointer features D_(A,1), D_(A,2),        D_(A,k), D_(A,m) on the curtain CUR1,    -   capturing a digital image FRAME1, which comprises a sub-image        SD_(A,k) of a pointer feature D_(A,k), and    -   determining the position of the pointer feature D_(A,k) from the        position of the sub-image SD_(A,k) by triangulation, and    -   comparing the determined position z(t₁) of the pointer feature        D_(A,k) with a reference position z(t₀).

The reference image FRAME0 may be captured e.g. by the imaging unit 100.In an embodiment, the reference image FRAME0 may also be provided by acomputer simulation from a numerically defined reference shape of thecurtain CUR1. The reference shape of the curtain CUR1 may be e.g. planaror curved. The reference image FRAME0 may be provided without using animaging unit. The reference position REF1_(k) may be provided bycomputer simulation without using an imaging unit.

Referring to FIG. 8 d, the field of view of the monitoring unit 400 maybe temporarily obstructed by an object O1, which is different from thecurtain CUR1. The object may be called e.g. as a foreign object. Inparticular, the object O1 may be a component of the apparatus 900, ahuman operator of the apparatus 900, or an object held by a humanoperator (e.g. a tool).

The position of a pointer feature D_(A,k) projected on the surface ofthe foreign object O1 may substantially deviate from the position ofsaid feature D_(A,k) in the reference situation. Consequently, it may bedifficult to determine which one of several sub-images appearing in thedigital image FRAME1 is the sub-image of the feature D_(A,k). Thedisplacement of the sub-image SD_(A,k) of said pointer feature D_(A,k)may so large that it may be difficult to determine the identity of thecorresponding feature D_(A,k).

Referring to FIG. 9 a, the projecting unit 200 may be arranged toproject a pattern PTRN1 on the curtain, wherein the pattern maysimultaneously comprise two or more dissimilar sub-patterns G₁, G₂,G_(k), G_(m). FIG. 9 a shows, in a side view, the curtain CUR1 and aplurality of sub-patterns G₁, G₂, G_(k), G_(m), which have differentshapes. For example, each sub-pattern may comprise e.g. two or morespots having characteristic positions and/or distances with respect toeach other. A first sub-pattern G_(k) may be located between a secondsub-pattern G₂ and a third sub-pattern G_(m), wherein the shape of thefirst sub-pattern G_(k) may be different from the shape of the secondsub-pattern G₂ and different from the shape of the third sub-patternG_(m). Thus, the identity (k) of the first sub-pattern G_(k) may bedetermined by analyzing the shape of the sub-image SG_(k) of the firstsub-pattern G_(k). The identity (k) of the first sub-pattern G_(k) maybe determined by determining whether the shape of the sub-image SG_(k)of the first sub-pattern G_(k) matches with the shape of a referenceimage.

The identity of each sub-pattern G₁, G₂, G_(k), G_(m) may be determinedbased on the characteristic shape of the sub-pattern G₁, G₂, G_(k),G_(m). The sub-pattern G₁ may comprise a feature D_(A,1), thesub-pattern G₂ may comprise a feature D_(A,2), the sub-pattern G_(k) maycomprise features D_(A,k), D_(B,k), D_(C,k) and the sub-pattern G_(m)may comprise a feature D_(A,m). The pointer features D_(A,1), D_(A,2),D_(A,k), D_(A,m) may be substantially similar, but each pointer featureD_(A,1), D_(A,2), D_(A,k), D_(A,m) may still be associated with adifferent identity because they belong to sub-patterns, which havedifferent shapes.

The identity of each sub-pattern G₁, G₂, G_(k), G_(m) may be determinedby comparing the shape of each sub-pattern with reference images. Forexample, when the sub-pattern G_(k) matches with a reference imageassociated with an identifier k, said sub-pattern G_(k) may also beassociated with said identifier k.

Referring to FIG. 9 b, a digital reference image FRAME0 may comprise animage of a pattern PTRN1 projected on the curtain CUR1 when the curtainCUR1 was in a reference state. The digital image FRAME0 may comprise aplurality of sub-images of the sub-patterns. The digital image FRAME0may comprise a sub-image SG₁ of the sub-pattern G₁, a sub-image SG₂ ofthe sub-pattern G₂, a sub-image SG_(k) of the sub-pattern G_(k), and asub-image SG_(m) of the sub-pattern G_(m). The sub-image SG_(k) maycomprise sub-images SD_(A,k), SD_(Bk), SD_(C,k), which may be images ofthe features D_(A,k), D_(B,k), D_(B,k) of the sub-pattern G_(k). Thedigital image FRAME0 may represent the situation at a time t₀. Thedigital image FRAME0 may represent the curtain CUR1 when it is inreference state. The reference image FRAME0 may comprise a plurality ofreference sub-images of the sub-patterns when the curtain CUR1 is inreference state.

Referring to FIG. 9 c, a digital image FRAME1 may comprise an image of apattern projected on the curtain CUR1 when the curtain CUR1 is deformedand/or displaced and/or when a sub-pattern is projected on a foreignobject O1. The sub-image SG_(k) may be an image of an unknownsub-pattern. The sub-image SG_(k) may be an image of a sub-pattern,which may be temporarily unknown until it is identified. The identity ofthe unknown sub-pattern of FIG. 9 c may be determined by comparing theunknown sub-image SG_(k) of FIG. 9 c with one or more known sub-imagesof the digital reference image FRAME0. The unknown sub-pattern may bedetermined to be associated with an identifier (k) when the sub-image ofthe digital image FRAME1 is detected to match with a reference sub-imageSG_(k) associated with said identifier (k).

The method may comprise:

-   -   projecting one or more features sub-patterns G₁, G₂, G_(k),        G_(m) on the curtain CUR1,    -   capturing a digital image FRAME1 which comprises a first        sub-image of a projected sub-pattern,    -   determining an identity for the first sub-image by determining        whether the first sub-image substantially matches with a        sub-image SG_(k) of a sub-pattern G_(k), which has known        identity (k), and    -   determining the position of the projected sub-pattern from the        position of the first sub-image by triangulation.

Referring to FIG. 9 d, the identity of several sub-patterns G₁, G₂,G_(k), G_(m) may be determined also in a situation where one or more ofthe sub-patterns G₁, G₂, G_(k), G_(m) are projected on the surface of aforeign object O1. For example, a sub-pattern G_(k) projected on thesurface of the object O1 may be recognized by determining whether anunknown sub-image of the image FRAME2 matches with the sub-image SG_(k)of the reference image FRAME0.

Referring to FIG. 10 a, the projecting unit 200 may be arranged toproject a pattern PTRN1 on the curtain CUR1, wherein the projectedpattern PTRN1 may comprise a plurality of sub-patterns G₁, G₂, . . . ,G_(k−2), G_(k−1), G_(k), G_(k+1), . . . G_(m). The pattern PTRN1 maycomprise e.g. a plurality of spatially separate pointer features, whichmay be located e.g. in pseudo random positions. Each sub-pattern G₁, . .. , G_(k−2), G_(k−1), G_(k), G_(k+1), . . . G_(m). may comprise e.g. twoor more pointer features. Each sub-pattern may be formed of e.g. 3, 4,5, or 6 dots. Each sub-pattern may be formed of six dots. Thereliability of identification may be increased by using a high number ofpointer features within a single sub-pattern. On the other hand, dataprocessing speed and/or spatial resolution may be improved by using asmall number of pointer features within a single sub-pattern

The pattern PTRN1 may be projected on the curtain CUR1 e.g. by using acombination of a laser beam LB0 and a holographic element 220.

The pattern PTRN1 may be e.g. a speckle pattern generated by diffractinglaser light LB0. Each sub-pattern G₁, G₂, . . . , G_(k−2), G_(k−1),G_(k), G_(k+1), . . . G_(m) may comprise e.g. two or more spots. A firstsub-pattern G_(k) may comprise two or more spots, the number of spots ofthe second sub-pattern G_(k−1) may be greater than or equal to thenumber of spots of the first sub-pattern G_(k), and the number of spotsof the third sub-pattern G_(k+1) may be greater than or equal to thenumber of spots of the first sub-pattern G_(k). The first sub-patternG_(k) may be located between the second sub-pattern G_(k−1) and thethird sub-pattern G_(k+1).

When the distance L_(k) between a predetermined sub-pattern G_(k) andthe imaging unit 100 is increased, the sub-image SG_(k) in the digitalimage FRAME1 may be displaced and enlarged, when compared with thesub-image SG_(k) in the reference image FRAME0. When the distance L_(k)between a predetermined sub-pattern G_(k) and the imaging unit 100 isdecreased, the sub-image SG_(k) in the digital image FRAME1 may bedisplaced and shrunk, when compared with the sub-image SG_(k) in thereference image FRAME0. Thus, a digital image FRAME1 may comprise afirst sub-image SG_(k) of a sub-pattern G_(k), and the reference imageFRAME0 may comprise a second sub-image SG_(k) of said sub-pattern G_(k)such that the first sub-image SG_(k) is displaced and scaled whencompared with the second sub-image SG_(k). The reference positionREF1_(k) may indicate the position of said second sub-image SG_(k). Thedistance L_(k) may sometimes vary so much that the first sub-imageSG_(k) is displaced so that a third sub-image SG_(k+1) of the digitalimage FRAME1 is located between the first sub-image SG_(k) and thereference position REF1_(k). The third sub-image SG_(k+1) may be asub-image of a second sub-pattern G_(k+1). In that case, it may bedifficult to associate a detected sub-image with the correspondingreference position.

For facilitating said association, the first sub-pattern G_(k) may belocally unique such that it does not match with any other sub-pattern inthe vicinity of the first sub-pattern G_(k). In an embodiment, the firstsub-pattern G_(k) may be globally unique such that it does not matchwith any other sub-pattern of the pattern PTRN1.

The distance L_(k) may be determined from the displacement Δu_(k) bytriangulation. The monitoring unit 400 may be arranged to associate adetected sub-image SG_(k) with a corresponding reference positionREF1_(k), in order to determine the displacement Δu_(k). However,associating a detected sub-image SG_(k) with the corresponding referenceposition REF1_(k) may be difficult e.g. when the sub-image SG_(k) isdisplaced so that another sub-image SG_(k+1) or SG_(k−1) is locatedbetween the sub-image SG_(k) and the reference position REF1_(k).

In an embodiment, a detected sub-image SG_(k) may be associated with thecorresponding reference position REF1_(k) based on the shape of thedetected sub-image SG_(k). The detected sub-image SG_(k) may beidentified based on the shape of the detected image SG_(k), and thecorresponding reference position REF1_(k) may be determined based on theidentity of the detected image SG_(k). In an embodiment, the sub-patternG_(k) may have a distinct shape so that the sub-image SG_(k) of thesub-pattern G_(k) may be identified based on the shape of thesub-pattern G_(k). In an embodiment, each sub-pattern G_(k) mayrepresent an identifying code.

In an embodiment, the sub-image SG_(k) may be unique so that thesub-image SG_(k) may be reliably and unambiguously associated with thecorrect reference position REF1_(k) based on the shape of the sub-imageSG_(k). In an embodiment, each sub-pattern G_(k−1), G_(k), G_(k+1) mayhave a unique shape. In an embodiment, the sub-pattern G_(k) may belocated between the sub-patterns G_(k−1), G_(k+1), wherein the shape ofthe sub-pattern G_(k) may be different from the shape of the sub-patternG_(k−1), and the shape of the sub-pattern G_(k) may be different fromthe shape of the sub-pattern G_(k+1).

The monitoring unit 400 may be arranged to detect that a digital imageFRAME1 comprises one or more sub-images SG_(k), SG_(k+1). A digitalimage FRAME1 may comprise one or more detected sub-images SG_(k),SG_(k+1). The monitoring system may be arranged to determine an identity(k) of a detected sub-image SG_(k). The monitoring unit 400 may bearranged to determine the identity of a detected sub-image SG_(k) bycomparing the detected sub-image SG_(k) with one or more candidatesub-images SG_(k−1), SG_(k), SG_(k+1). The monitoring unit 400 may bearranged to determine the identity of a detected sub-image SG_(k) bycomparing the shape of the detected sub-image SG_(k) with the shapes ofone or more candidate sub-images SG_(k−1), SG_(k), SG_(k+1). In anembodiment, each sub-pattern G_(k), G_(k+1) may have a unique shape suchthat the identity of the sub-image of each sub-pattern G_(k), G_(k+1)may be determined based on the shape of sub-image.

The monitoring unit 400 may be arranged to determine a referenceposition REF1_(k) corresponding to the detected sub-image SG_(k) basedon the identity of the detected sub-image SG_(k). The monitoring unit400 may be arranged to determine a reference position REF1_(k) of thedetected sub-image SG_(k) based on the identity of the detectedsub-image SG_(k).

For example, a plurality of identifiers ( . . . , k−1, k, k+1 . . . )and a plurality of reference positions REF1_(k−1), REF1_(k), REF1_(k+1)may be contained in table such that each identifier is associated with adifferent reference position.

In an embodiment, each sub-pattern may be a group of spots that may beunique within the whole pattern PTRN1 and can therefore be used touniquely determine the location in the pattern PTRN1.

Referring to FIG. 10 b, a given sub-pattern G_(k) does not need to beunique within the whole area of the pattern PTRN1. It may be sufficientwhen the sub-pattern G_(k) is locally unique. It may be sufficient ifthe sub-pattern G_(k) is unique within a portion BOX1 of the patternPTRN1. The portion BOX1 may be called e.g. as a search portion BOX1. Thepattern PTRN1 may optionally comprise other sub-patterns which areidentical to the sub-patterns G_(k), as long as the other identicalsub-patterns G_(k) are located outside the search portion BOX1. Thedimensions DIM1, DIM2 of the search portion BOX1 may be selected suchthat the identity of the sub-pattern G_(k) contained in the searchportion BOX1 may be determined with sufficient reliability even whenpattern PTRN1 comprises other identical sub-patterns outside the searchportion BOX1.

The search portion BOX1 may comprise e.g. sub-patterns G_(k−2), G_(k−1),G_(k), G_(k+1). The sub-pattern G_(k) may have a characteristic shape inorder to allow determining the correct reference position REF1_(k) witha sufficient reliability. The sub-pattern G_(k) may be different fromall other sub-patterns of the search portion BOX1 such that thesub-pattern G_(k) cannot be transformed into any other sub-pattern (e.g.G_(k−2), G_(k−1), or G_(k+1)) of the portion BOX1 only by using a lineartranslation operation in the direction DIR1 and a scaling operation. Thedirection DIR is parallel to the baseline BL0 of the monitoring unit400. The sub-pattern G_(k) may be different from all other sub-patternsof the search portion BOX1 such that the sub-pattern G_(k) cannot betransformed into any other sub-pattern (e.g. G_(k−2), G_(k−1), orG_(k+1)) of the portion BOX1 without using a rotation operation. Thesub-pattern G_(k) may be different from all other sub-patterns of thesearch portion BOX1 such that the sub-pattern G_(k) cannot betransformed into any other sub-pattern (e.g. G_(k−2), G_(k−1), orG_(k+1)) of the portion BOX1 by a linear translation operation in thedirection DIR1 without using a rotation operation.

In an embodiment, a first sub-pattern G_(k) of the search portion doesnot match with any of the other sub-patterns G_(k−2), G_(k−1), G_(k+1)within said search portion BOX1. Each sub-pattern within the searchportion BOX1 may be unique within the search portion BOX1 and cantherefore be used to uniquely determine a location within the searchportion BOX1. The sub-pattern G_(k−2) may comprise spots D_(A,k−2),D_(B,k−2). The sub-pattern G_(k−1) may comprise spots D_(A,k−1),D_(B,k−1). The sub-pattern G_(k) may comprise spots D_(A,k), D_(B,k).The sub-pattern G_(k+1) may comprise spots D_(A,k+1), D_(B,k+1).

Referring to FIG. 10 c, the direction DIR1 denotes a direction definedby the baseline BL0 of the monitoring unit 400, and the direction DIR2is perpendicular to the direction DIR1. The search portion BOX1 may havea dimension DIM1 in the direction DIR1, and the search portion BOX1 mayhave a dimension DIM2 in the direction DIR2. The sub-pattern G_(k) mayhave a dimension DIM3 in the direction DIR2. The dimension DIM1 of thesearch portion BOX1 may be selected e.g. based on the expected range ofdistances which need to be measured by the monitoring unit 400. Forexample, the dimension DIM1 may be e.g. in the range of 10% to 40% ofthe width w_(C) of the curtain CUR1. The dimension DIM2 of the searchportion BOX1 may be e.g. in the range of 100% to 200% of the dimensionDIM3. The direction DIR1 may be substantially parallel to the directionSX e.g. when the baseline BL0 of the monitoring unit 400 is horizontal.The DIR1 may be substantially parallel to the direction SY e.g. when thebaseline BL0 of the monitoring unit 400 is vertical.

Referring to FIG. 10 d, a sub-pattern G_(k) may comprise spots D_(A,k),D_(B,k), and a sub-pattern G_(k+1) may comprise spots D_(A,k+1),D_(B,k+1). The spots D_(A,k), D_(B,k) may define a line LIN_(AB,k), andthe spots D_(A,k+1), D_(B,k+1) may define a line LIN_(AB,k+1). Thedirection of the LIN_(AB,k) may be defined by an angle θ_(k) withrespect to a reference direction, e.g. with respect to the direction SX.The direction of the LIN_(AB,k+1) may be defined by an angle θ_(k+1)with respect to a reference direction, e.g. with respect to thedirection SX. The position of the spot D_(B,k) may be defined by theangle θ_(k) and by a distance e_(AB,k) with respect to the spot D_(A,k).The position of the spot D_(B,k+1) may be defined by the angle θ_(k+1)and by a distance e_(AB,k+1) with respect to the spot D_(A,k+1). Theangle θ_(k+1) may be substantially different from the angle θ_(k) suchthat the sub-pattern G_(k) cannot be transformed into the sub-patternG_(k+1) without performing a rotation operation.

Referring to FIG. 10 e, the baseline BL0 of the monitoring unit 400 mayhave an arbitrary orientation with respect to the curtain CUR1. Theorientation of the direction DIR1 of the baseline BL0 may be definede.g. by an angle φ1 with respect to a reference direction. The referencedirection may be e.g. the direction SX. The direction DIR1 may besubstantially parallel to the direction SX (i.e. φ1=0°), substantiallyparallel to the direction SY (i.e. φ1=90°), or the angle φ1 may bedifferent from 0° and different from 90°.

Referring to FIG. 10 f, the monitoring unit 400 may be arranged todetermine the distance L_(k) by using a first sub-pattern G_(k), whichpartially overlaps a second sub-pattern G_(k−1). For example, the firstsub-pattern G_(k) may comprise spots D_(A,k), D_(D,k) such that thefirst sub-pattern G_(k) does not comprise the spot D_(A,k−1). The secondsub-pattern G_(k−1) may comprise spots D_(A,k−1), D_(D,k) such that thesecond sub-pattern G_(k−1) does not comprise the spot D_(A,k). In otherwords, the first sub-pattern G_(k) and the second sub-pattern G_(k−1)may comprise a common spot D_(D,k). For example, the first sub-patternG_(k) may comprise the spots D_(A,k), D_(B,k), D_(C,k), D_(D,k) suchthat first sub-pattern G_(k) does not comprise the spots D_(A,k−1),D_(B,k−1), D_(C,k−1). The second sub-pattern G_(k−1) may comprise thespots D_(A,k−1), D_(B,k−1), D_(C,k−1), D_(D,k) such that the secondsub-pattern G_(k−1) does not comprise the spots D_(A,k), D_(B,k),D_(C,k). Also in this case, the first sub-pattern G_(k) cannot betransformed into the second sub-pattern G_(k−1) only by a lineartransform operation and a scaling operation.

Referring to FIG. 10 g, an image FRAME1 may comprise one or moresub-images of the pointer features. The image FRAME1 may comprise one ormore sub-images SG₁, SG₂, . . . , SG_(k−2), SG_(k−1), SG_(k), SG_(k+1),. . . SG_(m) of the sub-patterns G₁, G₂, . . . , G_(k−2), G_(k−1),G_(k), G_(k+1), . . . G_(m). SD_(A,k−2) denotes the sub-image of thepointer feature D_(A,k−2). The image FRAME1 may comprise one or moresub-images SD_(A,k−2), SD_(B,k−2), SD_(A,k−1), SD_(B,k−1), SD_(A,k−1),SD_(B,k−1), SD_(A,k), SD_(B,k), SD_(A,k+1), SD_(B,k+1) of pointerfeatures D_(A,k−2), D_(B,k−2), D_(A,k−1), D_(B,k−1), D_(A,k−1),D_(B,k−1), D_(A,k), D_(B,k), D_(A,k+1), D_(B,k+1), SD_(A,k−2) may denotethe sub-image of the feature D_(A,k−2). SD_(A,k−1) may denote thesub-image of the feature D_(A,k−1). SD_(A,k) may denote the sub-image ofthe feature D_(A,k). SD_(A,k+1) may denote the sub-image of the featureD_(A,k+1). Δu_(k) may denote the displacement of the sub-image D_(A,k)with respect to the reference position REF1_(k).

Referring to FIG. 11 a, the monitoring unit 400 may be arranged toproject a pointer line PLIN0 on the curtain CUR1. The projected patternPTRN1 may be a pointer line PLIN0.

The baseline BL0 of the monitoring unit 400 may be e.g. substantiallyparallel to the direction SY. The pointer line PLIN0 may have adimension h_(k) in the direction SY.

Referring to FIG. 11 b, a reference image FRAME0 may comprise asub-image SPLIN0 of the projected line PLIN0 when the curtain CUR1 is ina reference state.

SU and SV denote orthogonal directions of the image space. The directionSU may correspond to the direction SX of the real space, and thedirection SV may correspond to the direction SY of the real space.

Referring to FIG. 11 c, a digital image FRAME1 may comprise a sub-imageSPLIN0 of the projected line PLIN0 when the curtain CUR1 is in adeflected and/or deformed state. Δv_(k) may denote a displacement of thesub-image SPLIN0 with respect to a reference position REF1_(k).Displacement values Δv may be determined at several different locationsof the sub-image SPLIN0, corresponding to the situation prevailing at atime t₁. The displacement values Δv may be determined by analyzing thedigital image FRAME1. The displacement values Δv may be determined bycomparing the sub-image SPLIN0 of the digital image FRAME1 with thesub-image SPLIN0 of the reference image FRAME0. Deflection of thecurtain CUR1 at several different positions may be determined from thedisplacement values Δv by triangulation.

In particular, the images of the projected line PLIN0 may be captured byusing light which is specularly reflected from the front surface FS1and/or back surface BS1 of the curtain CUR1. In that case the curtainmay be very thin and/or it does not need to comprise light-scatteringheterogeneous microstructures.

FIG. 12 shows, by way of example, a data processing system 901 of thecurtain coating apparatus 900. The curtain coating apparatus 900 maycomprise the data processing system 901. The data processing system 901may be arranged to e.g. measure data indicative of the state of thecurtain CUR1. The data processing system 901 may be arranged to e.g.analyze data provided by the imaging unit 300. The data processingsystem 901 may be arranged to e.g. process data provided by themonitoring unit 400. The data processing system 901 may be arranged toe.g. control operation of the apparatus 900 based on measured data.

The system 901 may optionally comprise a velocity sensor 560 formeasuring the velocity v₁ of the primary web WEB0 and/or for measuringthe velocity of the coated web WEB1. The velocity sensor 560 may beomitted e.g. when the velocity v₁ is known and/or constant.

The system 901 may comprise the monitoring unit 400, which may bearranged to provide position data indicative of the displacement and/ordeformation of the material layer monitored by the unit 400.

The system 901 may optionally comprise a memory MEM1 for storingreference data REFDATA1. For example, the memory MEM1 may comprisereference data REFDATA1 for recognizing sub-patterns projected on thecurtain. The reference data REFDATA1 may comprise e.g. a reference imageFRAME0. The reference data REFDATA1 may comprise coordinates, whichdefine one or more reference positions REF1_(k).

The system 901 may optionally comprise an imaging unit 300 for capturingdigital images of the curtain CUR1. Digital images FRAME2 may be storede.g. in a memory MEM5. The system 901 may comprise the memory MEM5.

The system 901 may optionally comprise a flow control unit 511 foradjusting/setting the flow rate of coating material ADH0 flowing througha nozzle of the distributing unit 510.

The system 901 may optionally comprise a temperature control unit 512for controlling temperature of the coating material ADH0 flowing througha nozzle of the distributing unit 510.

The system 901 may optionally comprise an actuator 514 for mechanicallyadjusting a nozzle of the distributing unit 510. The actuator 514 may bearranged to adjust e.g. an internal dimension of the nozzle, theposition of the nozzle and/or the orientation of the nozzle.

The system 901 may optionally comprise a flow control unit 522 foradjusting one or more gas flow rates of the stabilizing unit 520.

The system 901 may optionally comprise a velocity control unit 562 foradjusting, setting and/or controlling the velocity v₁ of the web WEB0.

The system 901 may optionally comprise a memory MEM2 for storing dataDATA1 indicative of the displacement and/or deformation of the curtainCUR1. The data DATA1 may comprise distance information (z(t),L_(k)(t)).The data DATA1 may comprise shape information. For example, the DATA1may comprise shape data, which defines the measured shape of the curtainCUR1.

The system 901 may optionally comprise a memory MEM3 for storingcomputer program code PROG1, which when executed by one or moreprocessors may cause the system 901 to monitor the displacement and/ordeformation of the curtain CUR1. The computer program code PROG1 maycause the system 901 to control operation of the apparatus 900 based onthe data DATA1.

The system 901 may optionally comprise a memory MEM4 for storingoperating parameters PARA1 of the apparatus 900.

The system 901 may optionally comprise a memory MEM5 for storing digitalimages FRAME2.

The system 901 may optionally comprise a memory MEM6 for storingsurveillance data DATA2. The surveillance data DATA2 may e.g. indicatethat that a foreign object O1 blocked the view of the monitoring unit400 during the coating. The surveillance data DATA2 may e.g. indicate atime t₃ when the foreign object O1 blocked the view of the monitoringunit 400. The surveillance data DATA2 may e.g. indicate a positionx′_(H,i), y′_(H,i) of an uninspected portion H_(i) of the coated webWEB1.

The system 901 may comprise a control unit CNT1, which may comprise oneor more data processors for monitor the displacement and/or deformationof the curtain CUR1. The system 901 may comprise a control unit CNT1,which may comprise one or more data processors for controlling operationof the apparatus 900 based on the data DATA1.

The monitoring unit 400 may comprise one or more data processors fordetermining position by triangulation. In an embodiment, one or moredata processors may be located in the vicinity of the image sensor 110of the imaging unit 100. In an embodiment, one or more data processorsmay be remote from the image sensor 110 of the imaging unit 100.

One or more operating parameters PARA1 of the apparatus 900 may beadjusted based on the distance information provided by the monitoringunit 400. The operating parameters PARA1 may include e.g. the flow rateof the coating material ADH0 via the distributing unit 510, a dimensionof a nozzle of the distributing unit 510, the position and/ororientation of the distributing unit 510, a gas flow rate of thestabilizing unit 520. For example, the velocity v1 of the primary webWEB0 may be adjusted based on the distance information provided by themonitoring unit 400. For example, the velocity v1 of the primary webWEB0 may be adjusted substantially to a maximum value where theamplitude of deflection of the curtain CUR1 does not exceed apredetermined limit.

Referring to FIG. 13, the curtain CUR1 may be optionally considered tohave two or more non-overlapping zones ZONE1, ZONE2, ZONE3, ZONE4. Thezones may also be called e.g. as portions. The apparatus 900 may bearranged to measure a position value z(x₁,t₁) indicative of the positionof the first zone ZONE1 in the direction SZ at a time t₁. The apparatus900 may be arranged to measure a position value z(x₂,t₁) indicative ofthe position of the second zone ZONE2 in the direction SZ. The apparatus900 may be arranged to measure a position value z(x₃,t₁) indicative ofthe position of the third zone ZONE3 in the direction SZ. The apparatus900 may be arranged to measure a position value z(x₄,t₁) indicative ofthe position of the fourth zone ZONE4 in the direction SZ.

The apparatus 900 may be arranged to measure a coordinate value z(t)indicative of the position of each zone. The number of zones may be e.g.in the range of 4 to 20.

The image sensor 110 may comprise a two-dimensional array of detectorpixels, wherein the array may have e.g. N_(HPIX) columns. The integervalue N_(HPIX) may be e.g. in the range of 200 to 20000. The valueN_(HPIX) may be e.g. equal to 640, 800, 1024, 1152, 1280, 1360, 1366,1400, 1440, 1600, 1680, 1920, 2048, 2560, 3840, 4096, 7680, 8192, or15360. An image FRAME1 provided by the image sensor 110 may have aresolution according to one or more of the following standards: VGA,SVGA, WSVGA, XGA, WXGA, SXGA, HD, SXGA, WSGA, HD+, UXGA, WSZGA+, FHD,WUXGA, QWXGA, WQHD, WQXGA, 4K, 8KUHD, for example. The number of thezones may be e.g. in the range of 1 to N_(HPIX)/2.

Referring to FIG. 14 a, the produced web WEB1, WEB3 may comprise one ormore defect portions F_(j), F_(j+1), F_(j+2), F_(j+3), . . . . Theposition of a defect portion F_(j) may be specified e.g. by providingcoordinates x′_(j), y′_(j) with respect to a reference point REF2. Thedefect portion F_(j) may have a width Δx′_(j) and a height Δy′_(j). Thedefect portion F_(j) may be e.g. a portion where the thickness d₁ ofcoating layer ADH1 is lower than a first threshold value. The defectportion F_(j) may be e.g. a portion where the thickness d₁ of thecoating layer ADH1 is substantially equal to zero, i.e. the coatinglayer is missing. The defect portion F_(j) may be e.g. a portion wherethe thickness d₁ of coating layer ADH1 is higher than a second thresholdvalue. The defect portion F_(j) may be e.g. a portion where the spatialvariations of the thickness d₁ of coating layer ADH1 are too high (thismay degrade visual appearance of a transparent label).

The shapes of the actual defective areas of the web may deviate from arectangular form, but the position and size of each defect portionF_(j), F_(j+1), F_(j+2), F_(j+3) may be defined by a substantiallyrectangular boundary.

The location of a portion F_(j) may be associated with a coating time(e.g. t₁) when the curtain CUR1 exhibited excessive fluctuations. Thelocations of the defect portions F_(j), F_(j+1), F_(j+2), F_(j+3), . . .may be determined e.g. from the distance information provided by themonitoring unit 400. The location of a portion F_(j) may be associatedwith a production time t₁ when the distance L_(k) between a pointerfeature D_(A,k) and the imaging unit 200 was outside a predeterminedrange. The apparatus 900 may be configured to provide defect data DATA1,which specifies the locations of the defect portions F_(j), F_(j+1),F_(j+2), F_(j+3), . . . . The defect data DATA1 may be provided e.g. byanalyzing when the distance L_(k) between a pointer feature D_(A,k) andthe imaging unit 200 was outside a predetermined range.

The location x′_(j),y′_(j) of a defective portion F_(j) may beassociated with a coating time (e.g. t₂) when the curtain CUR1 has avoid VOID1. The lateral position of the defective portion F_(j) maycorrespond to the lateral position of the void VOID1 at the time t₂. Thedefect data DATA1 may be provided e.g. by analyzing when the curtainCUR1 has a void VOID1.

The produced web WEB1, WEB3 may comprise one or more non-inspectedportions H_(i), which were produced when a foreign object partially orcompletely blocked the field of view VIEW1 of the monitoring unit 400.The locations of the non-inspected portions H_(i) may be determined e.g.from the distance information provided by the monitoring unit 400. Thelocation x′_(H,i), y′_(H,i) of a portion H_(i) may be associated with acoating time (e.g. t₃) when the distance L_(k) between a pointer featureD_(A,k) and the imaging unit 200 was smaller than a predeterminedthreshold value. In other words, the location x′_(H,i),y′_(H,i) of aportion H_(i) may be associated with a time t₃ when the presence of aforeign object O1 was detected. The location x′_(H,i),y′_(H,i) may bedetermined by detecting the presence of the foreign object O1.

The presence of a foreign object O1 may be detected based on distanceinformation. The distance information may be provided by detecting theposition of a sub-image (SD_(A,k)) in an image frame FRAME1B, which maybe captured by the image sensor 110 at the time t₃.

The apparatus 900 may be configured to provide surveillance data DATA2indicative of temporary presence of an object O1 during the coating suchthat the surveillance data DATA2 is associated with one or more portionsH_(i) of the web WEB1, WEB3. The surveillance data DATA2 may be providede.g. by determining when the distance L_(k) between a pointer featureD_(A,k) and the imaging unit 200 was smaller than a predeterminedthreshold value.

The portion H_(i) may be defective or not defective. The surveillancedata DATA2 may merely indicate that information about the quality of theportion H_(i) is not available, because the presence of the foreignobject prevented monitoring the curtain during the time period when theportion H_(i) was processed. The portion H_(i) of the web WEB1, WEB3 maybe used e.g. for a less demanding application where the quality of theweb WEB1, WEB3 is not critical.

The method may comprise:

-   -   forming a curtain of coating material,    -   detecting a deformation of the curtain by using an imaging        device,    -   at least partially blocking a field of view of said imaging        device by a foreign object,    -   detecting the presence of said foreign object by monitoring a        distance between a sensor unit and said object, and    -   forming a defect map based on information obtained from the        imaging device and based on information about the presence of        said foreign object.

The method may comprise:

-   -   forming a curtain of coating material,    -   detecting a void in the curtain by using an imaging device,    -   at least partially blocking a field of view of said imaging        device by a foreign object,    -   detecting the presence of said foreign object by monitoring a        distance between a sensor unit and said object, and    -   forming a defect map based on information obtained from the        imaging device and based on information about the presence of        said foreign object.

The web WEB1, WEB3 may be associated with an identifier ID1. Theidentifier ID1 may be e.g. a code, which may be implemented e.g. byusing graphical alphanumeric code, a graphical barcode or an RFID tagattached to the WEB1, WEB3 or to a package of the web WEB1, WEB3.

The data DATA1, DATA2 may be associated with the web WEB1, WEB3 e.g.based on the identifier ID1.

The positions x′_(j), y′_(j) may be defined e.g. with respect to areference point REF2. The reference point REF2 may be e.g. apredetermined corner of the web WEB1, WEB3, or a marking produced on theweb WEB1, WEB3. The marking may be e.g. a hole, a printed dot or aprinted crosshair pattern. The location x′_(H,i),y′_(H,i) of a portionH_(i) may be defined e.g. with respect to the reference point REF2.

Referring to FIG. 14 b, the defect data DATA1 may be stored e.g. in amemory MEM2. The defect data DATA1 may be stored e.g. in a memory of aserver, and the defect data DATA1 may be accessed e.g. via the internet.The defect data DATA1 may be associated with the web WEB1, WEB3 e.g.based on an identifier ID1.

Referring to FIG. 14 c, the surveillance data DATA2 may be stored e.g.in a memory MEM6. The surveillance data DATA2 may be stored e.g. in amemory of a server, and the surveillance data DATA2 may be accessed e.g.via the internet. The surveillance data DATA2 may be associated with theweb WEB1, WEB3 e.g. based on an identifier ID1.

In an embodiment, the presence of voids VOID1, VOID2 may be detectedbased on the distance information. The illuminating light beam LB1 maybe transmitted through a void VOID1, VOID2 so that the first pointerfeature D_(A,k) is projected onto the surface 521 of the stabilizingunit 520. Thus, a void VOID1, VOID2 may be detected by determiningwhether the distance information (z(t),L_(k)(t)) matches with theposition of the surface 521.

In an embodiment, the presence of a single void VOID1, VOID2 may bedetected based on pixel value thresholding and based on distanceinformation (z(t),L_(k)(t)). The presence of the void may be detectedwith very high reliability when the pixel value thresholding indicatesthe presence of a void, and when the distance information simultaneouslyindicates the presence of a void at the same location.

The primary web WEB0 and the coated web WEB1 may be moving with respectto the contact line CL1 during the curtain coating. In an embodiment,also the primary web WEB0 may be deflected and/or deformed. In anembodiment, also the coated web WEB1 may be deflected and/or deformed.For example, the primary web WEB0 may be deflected and/or deformed dueto a pressure difference caused by the stabilizing unit 520. Forexample, the coating material ADH0 may comprise a solvent (e.g. water oralcohol), and one or more gas jets may be directed towards the coatedweb WEB1 in order to accelerate drying of the coated web WEB1. The gasjets may cause deflection and/or deformation of the coated web WEB1. Thecoated web WEB1 may be wetted by the coating material ADH0 to such adegree that the wetting may cause deflection and/or deformation of thecoated web WEB1. The coated web WEB1 may be wetted by the coatingmaterial ADH0 to such a degree that one or more holes are formed in thecoated web WEB1. In an embodiment, the primary web WEB0 and/or thecoated web WEB1 may be supported by an air cushion, which may allowsmall deflection and/or deformation of the primary web WEB0 and/or thecoated web WEB1.

Referring to FIG. 15, the apparatus 900 may comprise a monitoring unit400, which may be arranged to monitor deflections, deformations and/orvoids of the web WEB0 or WEB1. The distance information may be providedby projecting a primary pattern PTRN2 on primary web WEB0 or on thecoated web WEB1. The primary pattern PTRN2 may comprise one or morepointer features D′_(A,1), D′_(A,2), D′_(A,k), D′_(A,m). The primarypattern PTRN2 may comprise a first pointer feature D′_(A,k). Theapparatus 900 may be arranged to provide distance information(z(t),L_(k)(t)) by comparing the detected position (u_(k)(t₁)) of thefirst sub-image SD′_(A,k) of the first pointer feature D′_(A,k) with areference position REF1_(k). In an embodiment, the apparatus 900 maycomprise a first monitoring unit 400 for projecting a first primarypattern PTRN1 on a first material layer and a second monitoring unit 400for projecting a second primary pattern PTRN2 on a second materiallayer.

In an embodiment, the primary pattern PTRN1 may be simultaneouslyprojected on the curtain CUR1 and the on the coated web WEB1. In anembodiment, the primary pattern PTRN1 may be simultaneously projected onthe curtain CUR1 and the on the primary web WEB0. In an embodiment, theprimary pattern PTRN1 may be projected on the primary web WEB0. In anembodiment, the primary pattern PTRN1 may be projected on the coated webWEB1. A monitoring unit 400 may be arranged to detect whether the movingweb WEB0, WEB1 has one or more holes. The monitoring unit 400 may bearranged to detect holes e.g. by pixel value thresholding from a digitalimage of the web WEB0, WEB1.

A monitoring unit 400 may be arranged to monitor deflections,deformations and/or voids of a moving web WEB0, WEB1, WEB2, or WEB3during producing a label web WEB1 or a label laminate web WEB3.

The velocity of the web monitored by the unit 400 may be e.g. in therange of 0.5 m/s to 5 m/s. The first image FRAME1 may be captured byusing the imaging optics 120, and the distance L_(NORM) between theimaging optics 120 and the primary pattern PTRN1 projected on the webmay be e.g. in the range of 1.0 m to 4.0 m. The ratio of the width ofthe web to the distance L_(NORM) between the imaging optics 120 and theprimary pattern PTRN1 may be e.g. in the range of 0.5 to 1.2.

The relatively large distance L_(NORM) may minimize the risk ofcontaminating the optics with the coating material and/or in order toprovide space for maintenance operations. For example, a person O1 mayenter to the space between the monitoring unit 400 and the web in orderto perform a maintenance operation.

The images of the pointer features may be slightly blurred e.g. due todefocusing, image aberrations and/or motion blurring. The size of thepointer features may be rather large in order to facilitate reliabledetection of the pointer features by using the imaging unit 100. Forexample, the smallest dimension (e.g. height, width or diameter) of thepointer features D_(A,k) may be e.g. greater than 0.001 times the widthw_(WEB) of the web monitored by the unit 400. For example, the smallestdimension (e.g. height, width or diameter) of the pointer featuresD_(A,k) may be e.g. greater than 5 times the thickness of the webmonitored by the unit 400.

In an embodiment, the image FRAME1 captured by the imaging unit 100 doesnot need to reproduce surface roughness of the web. The web may move adistance e₁ during the optical exposure time period of the image FRAME1.The pointer feature D_(A,k), may have a height h_(k) in the direction ofmovement of the web. In an embodiment, the distance e₁ travelled by theweb during the optical exposure time period may be e.g. greater than orequal to 50% of the height h_(k). In an embodiment, the distance e₁travelled by the web during the optical exposure time period may be e.g.greater than or equal to the thickness of the web monitored by the unit400.

The web has edges EDG1, EDG2. The edges EDG1, EDG2 may be substantiallyaligned with the direction of movement of the web. The stationalylocation reference REF0′ may have the same x-coordinate as the locationreference REF0. The location reference REF0 may be used as the locationreference REF0′.

A monitoring unit 400 may be arranged to monitor instantanous lateralposition x_(EDG1), x_(EDG2) of an edge EDG1, EDG2 of the web. Themonitoring unit 400 may be arranged to monitor alignment, shrinkageand/or expansion of the web, by using information about the instantanouslateral position x_(EDG1), x_(EDG2) of an edge EDG1, EDG2 of the web.For example, a coating or laminating apparatus may be controlled basedon information about the instantanous lateral position of the edge.

In an embodiment, the instantanous lateral position x_(EDG1), x_(EDG2)of an edge EDG1, EDG2 may be monitored with a high reliability by ausing the combination of pixel value thresholding and distanceinformation. The instantanous lateral position x_(EDG1), x_(EDG2) of anedge EDG1, EDG2 may be monitored by:

-   -   illuminating the web WEB0, WEB1 with illuminating light LB3,    -   capturing a second image FRAME2 of the illuminated web WEB0,        WEB1, and    -   detecting the position X_(EDG1), X_(EDG2) of an edge EDG1, EDG2        of the web WEB0, WEB1 with respect to a location reference REF0″        by using the distance information (z(t),L_(k)(t)) and also by        using second information, wherein the second information is        obtained by comparing at least one pixel value of the second        image FRAME2 with a reference value.

Reliable detection of the edge may be useful e.g. when the web istransparent or translucent.

The method for detecting the position of the edge may comprisesimultaneously providing a first light beam LB1_(k) for projecting afirst pointer feature D_(A,k), and providing a second light beamLB1_(k+1) for projecting a second pointer feature D_(A,k+1), wherein anedge EDG1 of the web WEB0, WEB1 may be located such that the web WEB0,WEB1 intersects the first light beam LB1_(k) but does not intersect thesecond light beam LB1_(k+1).

In an embodiment, one or more pointer features D_(A,k) of the primarypattern PTRN1 may be projected on a material layer CUR1, WEB0, WEB1,WEB2, WEB3 such that the intensity of the illuminating light LB1 at theposition of the pointer feature D_(A,k) is lower than the intensity ofthe illuminating light LB1 impinging on a region, which surrounds thepointer feature D_(A,k). For example, one or more pointer featuresD_(A,k) may be dark spots.

Various aspects of the invention are illustrated by the followingexamples:

Example 1

A method, comprising:

-   -   forming a curtain (CUR1), which comprises a coating material        (ADH0),    -   forming a coated web (WEB1) by coating a moving primary web        (WEB0) with the coating material (ADH0), wherein the curtain        (CUR1) and the primary web (WEB1) meet at a contact line (CL1),    -   projecting a primary pattern (PTRN1) on a material layer (CUR1,        WEB0, WEB1), whose material moves with respect to the contact        line (CL1), wherein the primary pattern (PTRN1) comprises a        first pointer feature (D_(A,k)),    -   capturing a first image (FRAME1), which comprises a first        sub-image (SD_(A,k)) of the first pointer feature (D_(A,k)),    -   detecting the position (u_(k)(t₁)) of the first sub-image        (SD_(A,k)) in the first image (FRAME1), and    -   providing distance information (z(t),L_(k)(t)) by comparing the        detected position (u_(k)(t₁)) of the first sub-image (SD_(A,k))        with a reference position (REF1_(k)).

Example 2

The method of example 1, wherein the primary pattern (PTRN1) isprojected on the curtain (CUR1).

Example 3

The method of example 1 or 2 wherein the first pointer feature (D_(A,k))is projected by providing an illuminating light beam (LB1_(k)), whichimpinges on the material layer (CUR1), the first image (FRAME1) iscaptured by focusing light (LB2_(k)) reflected from the material layer(CUR1), the reflected light (LB2) is focused by imaging optics (120),the first pointer feature (D_(A,k)) and a principal point (PP1) of theimaging optics (120) define a line of sight (VLIN_(k)), an angle (γ_(k))between the direction of the illuminating light beam (LB1_(k)) and theline of sight (VLIN_(k)) is greater than or equal to 0.2 degrees, andthe distance information (z(t),L_(k)(t)) is determined by triangulationfrom the detected position (u_(k)(t₁)) of the first sub-image(SD_(A,k)).

Example 4

The method according to any of the examples 1 to 3 comprising:

-   -   simultaneously projecting a plurality of pointer features        (D_(A,k−2), D_(A,k−1), D_(A,k), D_(A,k+1)) on the material layer        (CUR1),    -   capturing a first image (FRAME1), which comprises sub-images        (SD_(A,k−2), SD_(A,k−1), SD_(A,k), SD_(A,k+1)) of the pointer        features (D_(A,k−2), D_(A,k−1), D_(A,k), D_(A,k+1)),    -   detecting the positions of the sub-images (SD_(A,k−2),        SD_(A,k−1), SD_(A,k), SD_(A,k+1)) in the first image (FRAME1),        and    -   determining distance information (z(t),L_(k)(t)) for several        different positions (x), which correspond to the positions of        the positions of the pointer features (D_(A,k−2), D_(A,k−1),        D_(A,k), D_(A,k+1)), wherein the distance information        (z(t),L_(k)(t)) is determined from the detected positions of the        sub-images (SD_(A,k−2), SD_(A,k−1), SD_(A,k), SD_(A,k+1)).

Example 5

The method of example 4, comprising simultaneously projecting a firstsub-pattern (G_(k)) and a second sub-pattern (G_(k+1)) on the materiallayer (CUR1), wherein the shapes of the first sub-pattern (G_(k)) andthe second sub-pattern (G_(k+1)) have been selected such that the firstsub-pattern (G_(k)) cannot be transformed into the second sub-pattern(G_(k)) by using only a linear translation operation and a scalingoperation.

Example 6

The method according to any of the examples 1 to 3 wherein the primarypattern (PTRN1) is a projected line (PLIN0), wherein a section of theprojected line (PLIN0) is used as the first pointer feature (D_(A,k)).

Example 7

The method according to any of the examples 1 to 6, comprising formingone or more illuminating light beams (LB1_(k)) by a holographic element(220), and directing the illuminating light beams (LB1_(k)) to thematerial layer (CUR1) so as to form the primary pattern (PTRN1).

Example 8

The method according to any of the examples 1 to 7 comprising adjustingone or more operating parameters (PARA1) of said coating based on thedistance information (z(t),L_(k)(t)).

Example 9

The method according to any of the examples 1 to 8 comprising:

-   -   illuminating the curtain (CUR1) by illuminating light (LB3),    -   capturing a second image (FRAME2), which comprises a sub-image        (SCUR1) of the curtain (CUR1), and    -   detecting a void (VOID1) of the curtain (CUR1) by comparing at        least one pixel value of the second image (FRAME2) with a        reference value.

Example 10

The method according to any of the examples 1 to 9 comprising producinga label web (WEB1) or a label laminate web (WEB3).

Example 11

The method according to any of the examples 1 to 10 comprisingdetermining defect data (DATA1) by monitoring the curtain (CUR1),wherein the defect data (DATA1) indicates the positions (x′_(j),y′_(j))of one or more defect portions (F_(j), F_(j+1)) of the coated web(WEB1).

Example 12

The method of example 11, comprising die-cutting the web (WEB1, WEB3)according to the defect data (DATA1).

Example 13

The method according to any of the examples 1 to 12, comprising:

-   -   capturing a documentation image (FRAME2) of the curtain (CUR1),        and    -   storing the documentation image (FRAME2) of the curtain (CUR1)        in a memory (MEM5) such that the documentation image (FRAME2) is        associated with one or more defect portions (F_(j)) in the        coated web (WEB1).

Example 14

The method according to any of the examples 1 to 13, comprising:

-   -   positioning an object (O1) in front of the curtain (CUR1) such        that a pointer feature (D_(A,k)) is projected on the object        (O1),    -   capturing a second image (FRAME1B) by using imaging optics (120)        such that the second image comprises a sub-image (SD_(A,k)) of        the pointer feature (D_(A,k)) in the second image (FRAME1B), and    -   providing surveillance data (DATA2) by detecting the position        (u_(k)(t₁)) of the sub-image (SD_(A,k)) of the pointer feature        (D_(A,k)) in the second image (FRAME1B).

Example 15

The method of example 14 comprising providing surveillance data (DATA2)indicative of temporary presence of an object (O1) during the coatingsuch that the surveillance data (DATA2) is associated with one or moreportions (H_(i)) of the coated web (WEB1).

Example 16

An apparatus (900), comprising:

-   -   one or more rolls (550) arranged to move a primary web (WEB0),    -   a distributor unit (510) arranged to form a curtain (CUR1),        which comprises a coating material (ADH0), and to form a coated        web (WEB1) by coating the primary web (WEB0) with the coating        material (ADH0) such that the curtain (CUR1) meets the primary        web (WEB0) at a contact line (CL1),    -   a projection unit (200) arranged to project a primary pattern        (PTRN1) on a material layer (ADH0, WEB0, WEB1), whose material        is arranged to move with respect to the contact line (CL1),        wherein the primary pattern (PTRN1) comprises a first pointer        feature (D_(A,k)),    -   an imaging unit (100) arranged to capture a first image        (FRAME1), which comprises a first sub-image (SD_(A,k)) of the        first pointer feature (D_(A,k)), and    -   one or more data processors (CNT1) configured to provide        distance information (z(t),L_(k)(t)) by comparing the position        (u_(k)(t₁)) of the first sub-image (SD_(A,k)) with a reference        position (REF1_(k)).

Example 17

The apparatus (900) of example 16 wherein the projection unit (200) isarranged to project the primary pattern (PTRN1) on the curtain (CUR1).

Example 18

The apparatus (900) of example 16 or 17 wherein the distance informationis provided by triangulation from the detected position (u_(k)(t₁)) ofthe first sub-image (SD_(A,k)).

Example 19

The apparatus (900) according to any of the examples 16 to 18 whereinthe primary pattern (PTRN1) comprises a plurality of distinct pointerfeatures (D_(A,k−2), D_(A,k−1), D_(A,k), D_(A,k+1)).

Example 20

The apparatus (900) according to any of the examples 16 to 19 whereinthe primary pattern (PTRN1) comprises a first sub-pattern (G_(k)) and asecond sub-pattern (G_(k+1)) wherein the shapes of the first sub-pattern(G_(k)) and the second sub-pattern (G_(k+1)) have been selected suchthat the first sub-pattern (G_(k)) cannot be transformed into the secondsub-pattern (G_(k)) by using only a linear translation operation and ascaling operation.

Example 21

The apparatus (900) according to any of the examples 16 to 18 whereinthe primary pattern (PTRN1) is a projected line (PLIN0).

Example 22

The apparatus (900) according to any of the examples 16 to 21 whereinthe primary pattern (PTRN1) is projected by using a holographic opticalelement (220).

Example 23

The apparatus (900) according to any of the examples 16 to 22 whereinthe operation of the apparatus (900) is controlled based on the distanceinformation (z(t),L_(k)(t)).

Example 24

The apparatus (900) according to any of the examples 16 to 23 comprising

-   -   a light source (600) arranged to provide illuminating light        (LB3), and    -   an imaging unit (300) arranged to capture a second image        (FRAME2, SCUR1) of the curtain (CUR1), and    -   one or more processors (CNT1) configured to detect a void        (VOID1) of the curtain (CUR1) by comparing at least one pixel        value of the second image (FRAME2) with a reference value.

Example 25

The apparatus (900) according to any of the examples 16 to 24, whereinthe apparatus (900) is arranged to produce a label web (WEB1) or a labellaminate web (WEB3).

Example 26

The apparatus (900) according to any of the examples 16 to 25,comprising one or more processors (CNT1) configured to determine defectdata (DATA1) by monitoring the curtain (CUR1), wherein the defect data(DATA1) indicates the positions (x′_(j),y′_(j)) of one or more defectportions (F_(j), F_(j+1)) of the coated web (WEB1).

Example 27

The apparatus (900) according to any of the examples 16 to 26,comprising:

-   -   an imaging unit (300) arranged to capture a documentation image        (FRAME2) of the curtain (CUR1), and    -   a memory (MEM5) for storing the documentation image (FRAME2) of        the curtain (CUR1) such that the documentation image (FRAME2) is        associated with one or more defect portions (F_(j)) in the        coated web (WEB1).

Example 28

The apparatus (900) according to any of the examples 16 to 27, whereinan object can be positioned between the curtain (CUR1) and the imagingunit (100) such that a pointer feature (D_(A,k)) is projected on theobject (O1), wherein the imaging unit (100) is arranged to capture asecond image (FRAME1B) such that the second image comprises a sub-image(SD_(A,k)) of the pointer feature (D_(A,k)), and one or more processorsare arranged to provide surveillance data (DATA2) by detecting theposition (u_(k)(t₁)) of the sub-image (SD_(A,k)) of the pointer feature(D_(A,k)) in the second image (FRAME1B).

Example 29

An apparatus (900), configured to:

-   -   form a curtain (CUR1), which comprises a coating material        (ADH0),    -   form a coated web (WEB1) by coating a moving primary web (WEB0)        with the coating material (ADH0),    -   project a primary pattern (PTRN1) on the curtain (CUR1), wherein        the primary pattern (PTRN1) comprises a first pointer feature        (D_(A,k)),    -   capture a first image (FRAME1), which comprises a first        sub-image (SD_(A,k)) of the first pointer feature (D_(A,k)),    -   detect the position (u_(k)(t₁)) of the first sub-image        (SD_(A,k)) in the first image (FRAME1), and    -   provide distance information (z(t),L_(k)(t)) by comparing the        detected position (u_(k)(t₁)) of the first sub-image (SD_(A,k))        with a reference position (REF1_(k)).

Example 30

A computer program (PROG1) comprising computer program code configuredto, when executed on at least one processor (CNT1), cause an apparatusor a system to:

-   -   form a curtain (CUR1), which comprises a coating material        (ADH0),    -   form a coated web (WEB1) by coating a moving primary web (WEB0)        with the coating material (ADH0),    -   project a primary pattern (PTRN1) on the curtain (CUR1), wherein        the primary pattern (PTRN1) comprises a first pointer feature        (D_(A,k)),    -   capture a first image (FRAME1), which comprises a first        sub-image (SD_(A,k)) of the first pointer feature (D_(A,k)),    -   detect the position (u_(k)(t₁)) of the first sub-image        (SD_(A,k)) in the first image (FRAME1), and    -   provide distance information (z(t),L_(k)(t)) by comparing the        detected position (u_(k)(t₁)) of the first sub-image (SD_(A,k))        with a reference position (REF1_(k)).

Example 31

A computer program product embodied on a non-transitory computerreadable medium (MEM3), comprising computer program code (PROG1)configured to, when executed on at least one processor, cause anapparatus or a system to:

-   -   form a curtain (CUR1), which comprises a coating material        (ADH0),    -   form a coated web (WEB1) by coating a moving primary web (WEB0)        with the coating material (ADH0),    -   project a primary pattern (PTRN1) on the curtain (CUR1), wherein        the primary pattern (PTRN1) comprises a first pointer feature        (D_(A,k)),    -   capture a first image (FRAME1), which comprises a first        sub-image (SD_(A,k)) of the first pointer feature (D_(A,k)),    -   detect the position (u_(k)(t₁)) of the first sub-image        (SD_(A,k)) in the first image (FRAME1), and    -   provide distance information (z(t),L_(k)(t)) by comparing the        detected position (u_(k)(t₁)) of the first sub-image (SD_(A,k))        with a reference position (REF1_(k)).

Example 32

A combination of a web (WEB1, WEB3) and data (DATA1), wherein the web(WEB1, WEB3) has been produced by curtain coating, and the data (DATA1)comprises defect data, which indicates the positions (x′_(j),y′_(j)) ofone or more defect portions (F_(j), F_(j+1)) of the web (WEB1,WEB3).

Example 33

The combination of example 32 wherein the data (DATA1) further comprisessurveillance data (DATA2) indicative of temporary presence of an object(O1) during the curtain coating, wherein the surveillance data (DATA2)is associated with one or more portions of the web (WEB1,WEB3).

34. A method, comprising:

-   -   projecting a primary pattern (PTRN1) on a moving web (WEB0,        WEB1), wherein the primary pattern (PTRN1) comprises a plurality        of pointer features (D_(A,k−2), D_(A,k−1), D_(A,k), D_(A,k+1)),    -   capturing a first image (FRAME1), which comprises sub-images        (SD_(A,k−2), SD_(A,k), SD_(A,k+1)) of the pointer features        (D_(A,k−2), D_(A,k−1), D_(A,k), D_(A,k+1)),    -   detecting the positions of the sub-images (SD_(A,k−2), SD_(A,k),        SD_(A,k+1)), and    -   providing distance information (z(t),L_(k)(t)) by comparing the        detected position (u_(k)(t₁)) of a first sub-image (SD_(A,k))        with a reference position (REF1_(k)) of said first sub-image        (SD_(A,k)).

Example 35

The method of example 34 comprising measuring deflection and/ordeformation of the web (WEB0, WEB1) based on the detected positions ofthe sub-images (SD_(A,k−2), SD_(A,k−1), SD_(A,k), SD_(A,k+1)).

Example 36

The method of example 34 or 35 comprising:

-   -   illuminating the web (WEB0, WEB1) with illuminating light (LB3),    -   capturing a second image (FRAME2) of the illuminated web (WEB0,        WEB1), and    -   detecting a void (VOID1) of the web (WEB0, WEB1) by using the        distance information (z(t),L_(k)(t)) and also by using second        information, wherein the second information is obtained by        comparing at least one pixel value of the second image (FRAME2)        with a reference value.

Example 37

The method of example 34 or 35 comprising:

-   -   illuminating the web (WEB0, WEB1) with illuminating light (LB3),    -   capturing a second image (FRAME2) of the illuminated web (WEB0,        WEB1), and    -   detecting the position (x_(EDG1), x_(EDG2)) of an edge (EDG1,        EDG2) of the web (WEB0, WEB1) with respect to a location        reference (REF0′) by using the distance information        (z(t),L_(k)(t)) and also by using second information, wherein        the second information is obtained by comparing at least one        pixel value of the second image (FRAME2) with a reference value.

Example 38

The method of example 37 comprising simultaneously providing a firstlight beam (LB1_(k)) for projecting a first pointer feature (D_(A,k)),and providing a second light beam (LB1_(k+i)) for projecting a secondpointer feature (D_(A,k+1)), wherein an edge (EDG1) of the web (WEB0,WEB1) is located such that the web (WEB0, WEB1) intersects the firstlight beam (LB1_(k)) but does not intersect the second light beam(LB1_(k+1)).

Example 39

The method according to any of the examples 34 to 38, comprisingsimultaneously projecting a first sub-pattern (G_(k)) and a secondsub-pattern (G_(k+1)) on the web (WEB0, WEB1), wherein the shapes of thefirst sub-pattern (G_(k)) and the second sub-pattern (G_(k+1)) have beenselected such that the first sub-pattern (G_(k)) cannot be transformedinto the second sub-pattern (G_(k)) by using only a linear translationoperation and a scaling operation.

Example 40

The method according to any of the examples 34 to 39 comprising formingone or more illuminating light beams (LB1_(k)) by a holographic element(220), and directing the illuminating light beams (LB1_(k)) to thematerial layer (CUR1) so as to form the primary pattern (PTRN1).

Example 41

The method according to any of the examples 34 to 40 wherein thevelocity (v1) of the web (WEB0, WEB1) is in the range of 0.5 m/s to 5m/s.

Example 42

The method according to any of the examples 34 to 41 wherein the firstimage is captured by using imaging optics (120), and the distance(L_(NORM)) between the imaging optics (120) and the primary pattern(PTRN1) projected on the web (WEB0, WEB1) is in the range of 1.0 m to4.0 m.

Example 43

The method according to any of the examples 34 to 42 wherein the ratioof the width (w_(WEB)) of the web (WEB0, WEB1) to the distance(L_(NORM)) between the imaging optics (120) and the primary pattern(PTRN1) is in the range of 0.5 to 1.2.

Example 44

The method according to any of the examples 34 to 43 wherein thethickness of the web (WEB0, WEB1) is in the range of 0.01 mm to 0.5 mm,in particular in the range of 0.02 mm to 0.3 mm.

Example 45

The method according to any of the examples 34 to 44 wherein the web(WEB0, WEB1) is transparent or translucent.

Example 46

The method according to any of the examples 34 to 45 wherein the web(WEB0, WEB1) comprises paper and/or plastic, in particular polyesterand/or polypropylene.

Example 47

The method according to any of the examples 34 to 46 wherein the web(WEB0, WEB1) comprises an adhesive layer and/or an anti adhesion layer.

For the person skilled in the art, it will be clear that modificationsand variations of the devices and the methods according to the presentinvention are perceivable. The figures are schematic. The particularembodiments described above with reference to the accompanying drawingsare illustrative only and not meant to limit the scope of the invention,which is defined by the appended claims.

What is claimed is:
 1. A method, comprising: forming a curtaincomprising a coating material; forming a coated web by coating a movingprimary web with the coating material, wherein the curtain and theprimary web meet at a contact line; projecting a primary pattern on amaterial layer, wherein the material layer moves with respect to thecontact line, wherein the primary pattern comprises a first pointerfeature; capturing a first image comprising a first sub-image of thefirst pointer feature; detecting the position of the first sub-image inthe first image; and providing distance information by comparing thedetected position of the first sub-image with a reference position. 2.The method according to claim 1, wherein the primary pattern isprojected on the curtain.
 3. The method according to claim 1, whereinthe first pointer feature is projected by providing an illuminatinglight beam that impinges on the material layer, the first image iscaptured by focusing light reflected from the material layer, thereflected light is focused by imaging optics, the first pointer featureand a principal point of the imaging optics define a line of sight, anangle between the direction of the illuminating light beam and the lineof sight is greater than or equal to 0.2 degrees, and the distanceinformation is determined by triangulation from the detected position ofthe first sub-image.
 4. The method according to claim 1, furthercomprising: simultaneously projecting a plurality of pointer features onthe material layer; capturing a first image, which comprises sub-imagesof the pointer features; detecting the positions of the sub-images inthe first image, and determining distance information for severaldifferent positions, wherein the distance information is determined fromthe detected positions of the sub-images.
 5. The method according toclaim 1, further comprising: simultaneously projecting a firstsub-pattern and a second sub-pattern on the material layer, wherein theshapes of the first sub-pattern and the second sub-pattern have beenselected such that the first sub-pattern cannot be transformed into thesecond sub-pattern by using only a linear translation operation and ascaling operation.
 6. The method according to claim 1, wherein theprimary pattern is a projected line, and wherein a section of theprojected line is used as the first pointer feature.
 7. The methodaccording to claim 1, further comprising: forming at least oneilluminating light beam by a holographic element; and directing theilluminating light beams to the material layer so as to form the primarypattern.
 8. The method according to claim 1, further comprising:adjusting at least one operating parameter of said coating based on thedistance information.
 9. The method according to claim 1, furthercomprising: illuminating the curtain by illuminating light; capturing asecond image, which comprises a sub-image of the curtain; and detectinga void of the curtain by comparing at least one pixel value of thesecond image with a reference value.
 10. The method according to claim1, further comprising: producing a label web or a label laminate web.11. The method according to claim 1, further comprising: determiningdefect data by monitoring the curtain, wherein the defect data indicatesthe positions of at least one defect portion of the coated web.
 12. Themethod according to claim 11, further comprising: die-cutting the webaccording to the defect data.
 13. The method according to claim 1,further comprising: capturing a documentation image of the curtain; andstoring the documentation image of the curtain in a memory such that thedocumentation image is associated with one or more defect portions inthe coated web.
 14. The method according to claim 1, further comprising:positioning an object in front of the curtain such that a pointerfeature is projected on the object; capturing a second image by usingimaging optics such that the second image comprises a sub-image of thepointer feature in the second image, and providing surveillance data bydetecting the position of the sub-image of the pointer feature in thesecond image.
 15. The method according to claim 14, further comprising:providing surveillance data indicative of temporary presence of anobject during the coating such that the surveillance data is associatedwith at least one portion of the coated web.
 16. A method, comprising:projecting a primary pattern on a moving web, wherein the primarypattern comprises a plurality of pointer features, capturing a firstimage comprising sub-images of the pointer features; detecting thepositions of the sub-images; and providing distance information bycomparing the detected position of a first sub-image with a referenceposition of said first sub-image.
 17. The method according to claim 16,further comprising: measuring deflection of the web based on thedetected positions of the sub-images.
 18. The method according to claim16, further comprising: illuminating the web with illuminating light;capturing a second image of the illuminated web; and detecting an edgeof the web by using the distance information and also by using secondinformation, wherein the second information is obtained by comparing atleast one pixel value of the second image with a reference value.
 19. Anapparatus, comprising: at least one roll arranged to move a primary web;a distributor unit arranged to form a curtain, which comprises a coatingmaterial, and to form a coated web by coating the primary web with thecoating material such that the curtain meets the primary web at acontact line; a projection unit arranged to project a primary pattern ona material layer, whose material is arranged to move with respect to thecontact line, wherein the primary pattern comprises a first pointerfeature; an imaging unit arranged to capture a first image, whichcomprises a first sub-image of the first pointer feature, and at leastone data processors configured to provide distance information bycomparing the position of the first sub-image with a reference position.20. A computer program product embodied on a non-transitory computerreadable medium, comprising computer program code configured to, whenexecuted on at least one processor, cause an apparatus or a system to:form a curtain, which comprises a coating material; form a coated web bycoating a moving primary web with the coating material; project aprimary pattern on a material layer, whose material moves with respectto the contact line, wherein the primary pattern comprises a firstpointer feature; capture a first image, which comprises a firstsub-image of the first pointer feature; detect the position of the firstsub-image in the first image; and provide distance information bycomparing the detected position of the first sub-image with a referenceposition.